Intensity-Modulated and Image-Guided Radiotherapy

In this chapter, we will describe both theoretical and practical considerations for modern image-guided radiotherapy (IGRT) simulation, planning, and treatment delivery. Most of the concepts can be applied to all forms of highly conformal therapy, including 3D conformal radiotherapy (3D CRT), intensity-modulated radiotherapy (IMRT), and volumetric-modulated arc therapy (VMAT). We also describe practical steps and workflows that can be applied to specific treatment planning technique(s).

Definitions for Conformal and Image-Guided Therapy

Conformal therapy describes radiotherapy treatment that creates high-dose volume(s), shaped to closely “conform” to the desired target volumes and prescription doses while minimizing (as much as possible) the dose to critical normal tissues. As capabilities have improved, the definition of conformal therapy has expanded to incorporate the fact that high-quality conformal therapy is designed to conform to all target dose requirements (both target volume shape[s] and the potentially complex dose distributions desired inside each target) while minimizing the normal tissue doses. Although these features are the general aim of any radiotherapy treatment, normally the term conformal is applied to treatment plans in which (1) the target volumes are defined in three dimensions using contours drawn on many slices from a computed tomography (CT) or other imaging study; (2) multiple beam directions are used to cross-fire on the targets; (3) the individual beams are shaped or intensity modulated to create a dose distribution that conforms (in shape and dose) to the target volume shape(s) and desired dose levels; and (4) appropriate use is made of image guidance, accurate patient setup and immobilization, and management of motion and other changes to ensure accurate delivery of the planned dose distributions to the patient so that deviations from the planned treatment of the patient are minimized. Fig. 21.1 illustrates the conceptual difference between standard and conformal therapy: Fig. 21.1A shows a standard four-field box treatment for a given target volume that everyone would agree is a nonconformal approach, whereas Fig. 21.1B shows a basic conformal approach achieved with conformally shaped fields.

Fig. 21.1, (A) Four-field box treatment of the prostate. Contours shown include blue external contour, red prostate, dotted planning target volume, yellow (dotted) rectum, and green femoral heads. (B) Beam's-eye view of a nonaxial four-field conformal plan for prostate treatment. The green outlines show the conformally shaped multileaf collimator patterns for each field. Red is prostate, blue is femoral heads, yellow is bladder, and brown is rectum.

A number of different treatment planning techniques and various treatment delivery techniques are routinely used to perform clinical conformal therapy. Three-dimensional (3D) CRT was the first conformal therapy technique developed based on the use of 3D treatment planning and multiple cross-firing, carefully shaped fixed fields. A more recent planning method—inverse planning—involves creation of the radiotherapy plan using mathematical optimization techniques. Often, this inverse planning technique is used to define intensity-modulated beams, which are radiation beams with a complex intensity distribution rather than the “flat,” uniform intensity fields typically used for 3D CRT. The combination of inverse planning and intensity-modulated beams is called intensity-modulated radiotherapy (IMRT). In recent years, the combination of IMRT delivery and optimization methods with arc therapy, volumetric modulated arc therapy (VMAT), has become an important method for the delivery of conformal therapy. All of these conformal therapy delivery methods are greatly improved by (and usually require) the use of image-guided radiotherapy (IGRT) techniques to accurately position and set up the patient, using integrated megavoltage or kilovoltage diagnostic imaging, cone beam CT, radiofrequency beacons or radiographic fiducials, and other image-guidance methods. Active consideration of the patient's respiratory (or other) motion in planning is described by four-dimensional (4D) imaging and 4D planning, and active motion management strategies are used during treatment to address the motion issues.

In general, there can be many different combinations of technologies used to develop and implement sophisticated conformal therapy. Conformal therapy is defined by the kinds of dose distributions that are planned and delivered to the patient, not by the specific technique(s) that are used. For example, one often performs conformal therapy by using IMRT, but the fact that IMRT is used does not necessarily imply that the treatment is conformal.

A Short History of Conformal and Image-Guided Therapy

For many decades, it has been known that delivering a high dose to a tumor is critical for control of the tumor and that the probability of complications increases with radiation dose and volume of organ irradiated. The basic concept of conformal therapy was elucidated quite early: “Treat the tumor to a high dose while minimizing the dose to normal tissues.” However, it was not until the 1950s and 1960s that techniques recognizable as modern conformal therapy began to be developed. Pioneers in conformal therapy include Shinji Takahashi, who developed early multileaf collimators (MLC), automated (mechanical) conformal beam shaping, dynamic conformal treatments, orthogonal light beams to identify the machine isocenter, and 3D tumor models based on early tomography ; Harold Perry et al. in Detroit; and Proimos, Trump, and Wright at the Massachusetts Institute of Technology (MIT)–Lahey Clinic. Another early approach to conformal therapy, known as the Tracking Cobalt Project, was led by Green, Jennings, and others at the Royal Northern and Royal Free Hospitals in England. This was a series of mechanical, electrical, and, finally, computer-controlled treatment machines developed to track disease spread, particularly along lymph node chains. By 1980, the computer-controlled version of the tracking system was in clinical use, although it was acknowledged that the largest obstacle to the routine use of conformation therapy was treatment planning. Finally, the Joint Center for Radiation Therapy (JCRT) in Boston added computer control to a modern linear accelerator so that the treatment table, gantry, collimator, collimator jaws, dose rate, and other parameters could be controlled dynamically while the beam was in use. The JCRT achieved the delivery of what is now called “dynamic conformal therapy,” a modern basis for computer-controlled conformal therapy.

The introduction of CT in the early 1970s was key to the development of modern 3D planning, since a complete 3D description of the anatomy of each patient was crucial. CT-based treatment planning quickly became widespread and inhomogeneity-corrected dose calculations also became possible because CT provided the necessary electron-density maps of the patient. Other imaging data, including magnetic resonance imaging (MRI) and positron emission tomography (PET), also became available for use in planning in the mid-1980s.

With the widespread implementation of CT-based planning, it became possible to make use of continuing improvements in computer technology and new software developments to fully create 3D treatment planning systems that incorporated 3D graphics and the “beam's-eye view” (BEV), a 3D graphic reconstruction of the patient anatomy projected into the divergent geometry used by the x-rays in the radiation beam ( Fig. 21.2 ). Using BEV displays to select beam angles, design field shaping and evaluate coverage of tumor and sparing of normal tissues is perhaps one of the most effective concepts in the entire 3D planning paradigm. Routine clinical use of 3D radiation treatment planning (RTP) began in 1986 ; subsequently, many academic centers began development and then use of 3D planning systems in their clinics.

Fig. 21.2, Beam's-eye view of a brain tumor showing a multileaf collimator shaped to the violet target volume. Normal tissues include brainstem (white) , chiasm (green) , and optic nerves (red) .

The development of 3D treatment planning systems helped drive development of more sophisticated machinery for efficient treatment delivery. The first treatment machine designed specifically to perform computer-controlled conformal radiotherapy (CCRT), the Scanditronix MM50 Racetrack Microtron (Scanditronix AV, Uppsala, Sweden), was developed during this same time period. Among other unique features, this machine included a fully computerized control system and a computer-controlled multileaf collimator (MLC), consisting of two sets of thin tungsten leaves that were used to shape the radiation field. Virtually all other radiotherapy machines have since also implemented computer control systems and MLC systems.

The capabilities of computer control and MLC systems have made possible the delivery of complicated plans, including those that make use of modulated intensities (a beam with different intensities in different parts of the field). Intensity modulation created using multiple segments or dynamic MLC motions and computer plan optimization (inverse planning) have been integrated and called intensity-modulated radiotherapy (IMRT). The basic concepts of IMRT were described in 1987 by Brahme and a practical implementation was described by Bortfeld et al. soon after. The combination of the flexibility of computer-controlled IMRT delivery with sophisticated plan optimization techniques has made IMRT an extremely powerful tool that can be used to perform conformal therapy.

The initial commercial IMRT implementation by NOMOS in 1992 was a form of IMRT now called serial tomotherapy , in which patients were treated slice by slice (as with early CT scanners) by the machine rotating around the patient using a special multileaf collimator (MIMIC) that performed the intensity modulation. Within a few years, all major vendors had implemented MLC systems, with leaf widths varying from 1 cm to a few millimeters, that could perform IMRT using either dynamic motions of the MLC leaves (“DMLC”) or a number of static segments (called “SMLC”). A more sophisticated implementation of tomotherapy based on helical delivery of IMRT, helical tomotherapy, also became widely disseminated. In the last several years, a rotational MLC-based IMRT technique, originally described by Yu, has been implemented and widely disseminated. This technique, volumetric-modulated arc therapy, allows for treatment under dynamic changes of the gantry, MLC position, dose rate, and, in some solutions, collimator rotation.

Inverse planning was also developed substantially during this time. Though much of this optimization makes use of quadratic weighted sum cost functions and simple gradient-based search algorithms, there have also been developments of sophisticated cost functions and the use of more biologically related costs that make use of normal tissue control probability (NTCP) models and equivalent uniform dose (EUD). Though most inverse planning systems use the weighted sum cost functions, advanced multicriteria methods have been developed that more directly take into account the numerous optimization goals involved in a typical clinical radiotherapy treatment plan and prioritize clinical trade-offs. Most recently, there has been interest in knowledge-based planning (KBP) predictive models that estimate the dose distribution or dose-volume histogram (DVH) of a given patient using a model trained by extracting geometric and dosimetric features from similar high-quality plans and applying that knowledge to new patients. The use of KBP and other methods to automate treatment plans has become increasingly popular.

In addition to modernizing planning systems, one of the advances that made the conformal therapy revolution possible was the development of amorphous silicon flat panel imagers, which allowed effective electronic portal imaging verification of the accuracy of these newly conformal fields. This technology then was further expanded, first for kilovoltage (diagnostic quality) imaging and then to provide cone beam CT (CBCT) capability using kilovoltage imaging systems mounted directly on the treatment machine. The availability of these high-quality CBCT or kilovoltage imaging modalities directly on the treatment machine led to the development of IGRT, in which diagnostic imaging is used to correct patient setup and positioning for treatment every day. IGRT processes have greatly increased the delivery accuracy that can be routinely achieved and have led to the possibility of much smaller margins for setup errors. This improved confidence in targeting accuracy has made possible the development of stereotactic body radiotherapy (SBRT), which is now used routinely to give very high doses to well-localized targets in the liver, lung, and other sites using highly conformal treatment delivery performed with IGRT. The use of IMRT and the proper handling of patient motion, respiration, and other 4D issues have been pursued for many years and are still major threads of much current research and development. More recently, the addition of MR guidance for treatment using an integrated MRI and linear accelerator (LINAC) or cobalt unit has allowed for improved soft-tissue resolution for sites such as the abdomen and pelvis. This strategy is promising for difficult IGRT situations such as pancreas and liver and will continue to be developed over the coming years.

Issues for Clinical Use

A number of important clinical features are crucial for planning and delivery of high-quality conformal therapy. These issues should be carefully considered throughout the conformal therapy planning and delivery process.

Conformal therapy attempts to carefully conform the dose to the target(s); thus, accurate delineation of the target(s) and careful specification of the desired dose distribution(s) are crucial.
Patient immobilization, localization, and motion evaluation are crucial; setup accuracy and motion management must be considered throughout the process. Needing to increase target margins owing to positional variability will significantly decrease the advantages provided by conformal treatment.
Improvement of the clinical results achieved by conformal therapy compared with standard techniques depends on choosing the correct trade-offs between target coverage and normal tissue sparing. These choices must be made carefully and appropriately, and communication between physicians and planners must be clear, preferably documented in the electronic physician intent statement.
It is the quality and precision of the dose distribution delivered to the patient that is ultimately most important. The techniques used for planning and delivery (e.g., DMLC IMRT, VMAT, and 3D) are simply the means to achieve the desired planned dose distribution and rely on appropriate setup/IGRT/motion management to deliver properly.
The quality of the delivered dose distribution can be destroyed by respiration, setup error, or involuntary motions during treatment; thus, use of motion management techniques to eliminate or compensate for motion (e.g., active breathing control, respiratory gating, and tumor tracking) is essential for some clinical sites. The use of 4D CT for planning, and IGRT for setup and motion analysis, are all big contributors to the quality of the treatment.
It is also important to consider treatment delivery duration when developing a plan, since this impacts clinic throughput as well as patient tolerability. For example, if a patient will be treated with voluntary breath hold at exhale, the number of partial arcs and beams may be reduced for tolerability, with an acceptable small plan degradation. On the opposite end of complexity, palliative treatments with the intent to address pain should be as simple and quick to set up and deliver as clinically acceptable.

Planning for Conformal Therapy

Treatment planning is one of the most critical parts of the conformal therapy process. In this description, we include all preparatory aspects of the planning process, including many activities that occur outside the therapy planning system (TPS). Many treatment delivery issues (e.g., setup accuracy, patient motion, portal, and localization imaging) are briefly mentioned here and are more completely described later. Fig. 21.3 shows a schematic of the basic components of the planning process for both forward (interactive) and inverse (i.e., IMRT optimization) planning, with physician-specific steps in bold.

Fig. 21.3, Schematic chart of the basic components of the planning process, shown for both forward and inverse planning. CT, Computed tomography; MR, magnetic resonance; PET, positron emission tomography.

Positioning and Immobilization

One of the basic ideas of conformal therapy is to minimize the dose to normal tissues while conforming the dose to the target. Therefore, it is crucial to accurately position and immobilize the patient for each procedure in the planning and delivery process. One of the first clinical decisions to be made for each patient includes what position to use for the patient's treatment and whether any positioning and immobilization devices or aids will be used.

Basic patient positioning—including arms and legs and positioning of the patient's body as a whole (supine, prone, or in some other more unusual position)—depends mainly on two issues: (1) patient comfort and stability and (2) the beam directions that will be used. In most cases, conformal therapy plans make use of three or more beams that cross-fire on the target from a number of different angles arranged around the patient. Thus, the patient is typically positioned with both arms up (if the target is somewhere in the torso) or arms down (head and neck and brain targets). Superficial targets are typically positioned facing up to allow for their easy visualization. For most deep tumors, however, the cross-firing beam directions can be achieved with the patient in standard supine or prone positions, whichever is most stable and accurately set up. There have been studies of the benefits of various positioning decisions (e.g., prone vs. supine) for patients with prostate tumors ; there is some debate about the relative merits of the possible anatomic changes that occur for the prone versus supine position relative to other advantages and disadvantages for planning, daily setup, and respiratory motion-related stability.

The use of various types of so-called immobilization devices to help with patient positioning and immobilization for conformal therapy has run the entire gamut of possibilities, from the use of stereotactic head frames and other such devices that are physically attached to the patient's skull to other techniques that do not use any immobilization device. Early conformal therapy (1980s–1990s) often incorporated a foam cradle device to help position the patient. Currently, it is thought that more precision can be achieved without the use of the cradle devices in many situations. In the end, each clinic should document the setup accuracy that is achieved with its chosen methods for each clinical site so that the planning and delivery process can take proper account of the expected systematic and random setup uncertainties. The use of in-room imaging systems (e.g., diagnostic and megavoltage CBCT) has provided more detailed information about setup accuracy, making it possible to improve setup accuracy and minimize margins using IGRT setup.

In addition to considering patient positioning and immobilization, it is also important to decide and communicate what type of presimulation preparation to order (e.g., full bladder, empty rectum, nothing to eat or drink for 2 or 4 hours, and so on) and what type of motion management is preferred if relevant to the body site treated. Other considerations that should be decided and ordered include the use of bolus and/or intravenous or oral contrast (and at what timing). If the physician thinks about these considerations and places these detailed orders while evaluating the case at time of consult, it enhances the first-time quality of orders and, ultimately, the simulation images.

Imaging for Planning: Computed Tomography Simulation, Magnetic Resonance Imaging, and Other Cross-Sectional Imaging

The development of x-ray CT in the 1970s and its application to radiotherapy planning were absolutely crucial milestones in the development of conformal therapy techniques. Without the cross-sectional anatomic imaging provided by CT (or MRI), there was not enough anatomic knowledge about the tumor or normal anatomy to consider the use of highly conformal dose distributions. Once the detailed anatomic information provided by CT became available, it was clear that radiotherapy planning and treatment should make use of this new and detailed description of the patient to better spare normal tissues and more accurately deliver dose to the tumor. Conformal therapy is a logical response to the detailed information provided by CT.

Modern conformal therapy is always based on a 3D anatomic model of the patient, which is typically constructed from a CT scan of the involved region. Usually, a specific type of CT scan, the treatment planning scan or CT simulation, is obtained for use as the basis for treatment planning. Features of a basic treatment planning CT scan are listed in Box 21.1 . One specific treatment planning CT scan procedure often used is called CT simulation. The CT simulation usually consists of a treatment planning CT scan plus the delineation of a reference point within the CT dataset (to be used for later planning or setup procedures), followed by marking this reference point on the patient's skin. Sometimes, CT simulation software on the CT scanner workstation is used to define the beams for the treatment plan, though this activity is just simple treatment planning performed on the CT workstation.

Box 21.1

Treatment Planning Computed Tomography (CT) Scan

Use a flat tabletop to mimic the treatment machine couch. Make use of couch coordinates or other documentation of exact positioning if possible.
Use patient immobilization devices to position the patient in the same way as for treatment, including registration of the device with the couch top if available.
Scan extent should include any organs to be considered as organs at risk because planning typically requires delineation of entire organs to make use of biological effect data. For example, treatment anywhere in the thorax would typically require scanning from the apex of the lung to below the diaphragm so that the entire lung could be delineated.
Modern CT scanners are efficient enough that 1- to 3-mm-thick slices can be obtained throughout the entire region of the scan, though use of thicker slices outside the target region is possible. This can amount to several hundred CT slices per study.
Careful consideration must be given to the advantages and disadvantages of using contrast during the scans. Contrast can help identify organs but can also make accurate dose calculations difficult because it distorts the apparent electron density of the tissue, affecting dose calculations.
The treatment planning CT scan study is used for (1) target definition and delineation, (2) normal tissue delineation, (3) the overall anatomic model of the patient, (4) planning, (5) dose calculations based on electron density obtained from CT numbers, and (6) creation of digital reconstructed radiographs used to set up and verify patient position. Each of these uses may require some individualization of the CT scan protocol or use.
The CT scan protocol should be defined for each clinical site, including slice thicknesses, scan extent (top and bottom), reconstruction window size, use of contrast, patient positioning and immobilization, patient breathing control or instructions, and so on.
Motion management (consideration of how motion will be managed for CT scanning and for treatment) is an important part of the preparation for planning. It is appropriate to (1) control respiratory motion or (2) perform a 4D CT scan for patients who demonstrate any significant motion in the target region. It may be necessary to perform a quick scan to evaluate motion before deciding on the final planning scan protocol and methodology.

Developments in CT and treatment delivery technology have made the consideration of motion during CT scanning (and radiotherapy treatment) an important topic. 4D CT describes various techniques for obtaining CT data correlated with patient respiratory phase information so that the changes associated with respiration (or other motion) are displayed. For certain clinical sites (e.g., lung, breast, and upper abdomen), it is clear that consideration of respiratory motion is an important aspect of the initial imaging of the patient; thus, 4D CT is used to create an appropriate motion model of the patient for further planning and analysis. 4D CT, respiratory gating, active breathing control (ABC), or other methods for motion management are often used for many treatment sites, though which combinations of techniques and methods are most efficient and appropriate is not yet clear.

CT provides anatomic and electron density information that is critical for most treatment planning; it also provides a geometrically accurate basis for planning. However, it provides only anatomic information, not the physiologic and functional information that would be helpful for planning, and it has only limited amount soft-tissue contrast. MRI can provide complementary data, including excellent soft-tissue contrast and different kinds of physiologic information. In addition, functional magnetic resonance imaging (fMRI) studies can provide some of the functional information that has been unavailable until now. Other kinds of imaging also contain complementary or new information. PET and single-photon emission computed tomography (SPECT) provide functional and physiologic information that can be quite important in helping define target volumes and regions that should be included or excluded from the radiation fields. Which modalities, scans, tracers, and analysis methods should be used for specific features are well beyond the scope of this chapter. However, to quantitatively make use of any additional imaging modality for treatment planning, one should incorporate a number of important procedures into the imaging process, as listed in Box 21.2 .

Box 21.2

Features of Additional Imaging for Treatment Planning (4DCT, MRI, PET)

Use a flat tabletop to mimic the radiotherapy LINAC couch.
Use patient immobilization devices to position the patient as you would for treatment.
Ensure that the imaging protocol provides enough anatomic information to allow geometric registration of the new scan study with the base CT information.
Imaging protocol parameters should be optimized to provide the anatomic, physiological, or function data that are desired.
The scan protocol should be defined for each clinical site and protocol, including slice thicknesses, scan extent (top and bottom), scan orientation (nonaxial slices are possible in most non-CT methods), reconstruction window size, use of contrast, patient positioning and immobilization, patient breathing control or instructions, and so on. For MRI, many additional scan technique parameters must be defined.
CT scans should be performed in a manner that will allow accurate registration of other scan information (e.g., MRI and PET) with the CT scans.

4D CT, Four-dimensional computed tomography; LINAC, linear accelerator; MRI, magnetic resonance imaging; PET, positron emission tomography.

To use more than one imaging dataset for planning, the additional datasets will have to be registered geometrically to the original (base) CT dataset (as described later). It is important during the imaging process (1) to position and align the patient similarly for each of the imaging studies because this makes the registration process more straightforward and (2) to obtain all information necessary so that the dataset registration and fusion process can be performed quickly and accurately. If the patient geometry is different between scans, deformable registration may be useful to “align” datasets, but the accuracy of that alignment is still being worked out for each registration method and situation. Finally, after image registration, evaluation of the quality of registration and the resulting residual uncertainty is crucial, as described in the report from Task Group 132 of the American Association of Physicists in Medicine (AAPM).

Recently, advances in image acquisition and processing have allowed for progress in MRI-only treatment planning, enabling oncologists to take advantage of the superior soft-tissue contrast in MRI without the potential errors and inefficiencies associated with needing to register the images with a CT to provide density information for dose calculations. Several methods for MR-based pseudo-CT generation have been developed, including atlas-based and classification-based methods that assign electron densities based on a combination of several MR image sets, and hybrids of both methods. Further work is necessary to refine these methods and bring them to routine clinical use for specific situations, although much progress has been made for treatment of brain tumors in particular.

Prior to acquisition of treatment planning images at the time of simulation, important considerations include the need for contrast enhancement, motion management, and scan parameters for treatment planning and IGRT. A workflow for screening for contrast safety, ordering, and administration of IV contrast is necessary. As described elsewhere, motion management should be thought out for proper patient training and scheduling of simulation appointment duration. Finally, scan parameters should be specified to include enough anatomy for full range of motion for highly mobile tumors, for meaningful DVHs (e.g., include full lungs to assess V20), and to provide sufficient relevant anatomy for IGRT (e.g., include landmarks such as C2, last rib, or iliac crests for spine treatments to ensure proper superior-inferior alignment).

Anatomy for Treatment Planning

Treatment planning is a computer simulation of the process of radiotherapy treatment. It is based on creating a model of the patient inside the planning software, simulating radiation beams and the dose that those beams deliver to the patient. The definition of the virtual model of the patient—based on CT, MRI, and other types of imaging data—and how that anatomic model is used are crucial parts of the radiotherapy treatment planning process.

Three-Dimensional Anatomic Model

For conformal therapy planning, the representation of the patient used for treatment planning must be realistic and 3D. In general, the basic data used to define the anatomy come from a CT study, although MR-only treatment planning is being investigated in select situations. The anatomic model of the patient is based on this CT data and consists of a number of objects (structures) that delineate organs or other objects (e.g., target volumes) in 3D (or 4D; see the “Motion, Setup, and Four-Dimensional Anatomy” section). These structures can be defined by (1) a series of contours, (2) a 3D surface description, (3) a voxel-based description, or (4) a set of points distributed either randomly or on a grid. Methods used to delineate these structures are described in the “Structure Delineation and Contouring” section.

The 3D anatomic structures are crucial to the planning process. The various target and normal tissue structures describe the areas to be irradiated or to be spared, respectively, so that beams can be oriented and shaped. Graphic display of the 3D structures can also be used for geometric registration of the treatment planning anatomy to localize images obtained during simulation or treatment procedures. The external surface and any inhomogeneities are used for the dose calculation process. Most plan evaluation tools (e.g., DVHs) make use of these 3D structures for plan evaluation of the dose distribution.

Structure Delineation and Contouring

The delineation of the 3D anatomic objects (structures) used for planning and plan evaluation is one of the most important and time-consuming aspects of the entire conformal therapy process. Autosegmentation algorithms theoretically could improve efficiency but, thus far, they are of limited help. Accurately defining these contours is critical, since error or inaccuracy becomes a systematic error throughout the entire conformal therapy process. Errors in the process may come from sloppiness, not knowing what is being visualized on the image, limitations in the accuracy of the scan information (e.g., motion during the scan acquisition), interpolation of contours, and from many other problems. It is important that the 3D character of the objects being outlined is handled correctly: for example, sharp corners or spikes in a contour on just one slice are usually incorrect because such a structure will usually show related features on a number of images. To avoid this type of drawing problem, it is important to review all contours serially or to visualize the 3D shape of the object using coronal and sagittal planes so that any unrealistic “spikes” can be identified and edited. Standard nomenclature should be used for all structures, in agreement with the report from the AAPM Task Group 263: Standardizing Nomenclatures in Radiation Oncology, which provides very specific recommendations from a working group comprised of all major stakeholders including the AAPM, American Society for Radiation Oncology (ASTRO), European Society for Therapeutic Radiation Oncology (ESTRO), NRG Oncology, the Children's Oncology Group, Integrating Healthcare Enterprise in Radiation Oncology, and the Digital Imaging and Communications in Medicine Working Group. Standard structure names help to improve communication in each clinic as well as allow for aggregation and analysis of data from across institutions to fuel big data analysis and subsequent decision support tools.

Target Volume Definition and Margins

To plan and deliver conformal therapy, it is essential to accurately define the volumes that must receive high radiation doses—the “target volumes.” As described in detail in the International Commission on Radiological Units (ICRU) report ICRU-50, three kinds of target volumes are typically defined, as summarized in Table 21.1 .

TABLE 21.1

ICRU-50 Target Volume Definitions

Abbreviation	Name	Description
GTV	Gross tumor volume	Volume of macroscopic tumor that is visualized on imaging studies
CTV	Clinical target volume	Volume that should be treated to a high dose, typically incorporating both the GTV and volumes that are assumed to be at risk as a result of microscopic spread of the disease
PTV	Planning target volume	Volume that should be treated to ensure that the CTV is always treated, including considerations of systematic and random daily setup errors and inter- and intratreatment motion

ICRU, International Commission on Radiation Units and Measurements.

The gross target volume (GTV) is typically delineated by drawing the imaged tumor on each of the imaging studies that are available. CT is used often but, for many sites, MR and PET can be useful. When multiple imaging studies are available, the GTV can be drawn on each study; then, using dataset registration to geometrically align the different datasets (see the “Multiple Imaging Modalities: Dataset Registration and Fusion” section), one can combine or transfer the different GTV contours onto a single dataset. How to combine the various GTVs defined is the subject of ongoing research; however, typically, one will combine or take the union of all the defined GTVs to make sure that no gross tumor is missed within the final GTV. Depending on the quality of the registration, additional margin is sometimes warranted for registration error. Supplemental information including direct laryngoscopy or colonoscopy should also be incorporated if appropriate.

The definition of the clinical target volume (CTV) is probably the most important thing that the physician does in the conformal therapy process, because the CTV defines the region that should be treated with the prescribed dose. The CTV typically combines the GTVs plus any volumes that may contain microscopic disease that could not be imaged. The CTV depends on knowledge of the patterns of disease spread and incorporates any other clinical knowledge of the disease or the specific risks for spread that apply to the individual patient. The CTV is usually created by combining two kinds of information: (1) often, an expansion of the GTV by some margin (typically 0.5–1 cm in all directions, with extra expansion along longitudinal structures such as the esophagus, rectum, or muscle compartments) is used to account for microscopic invasion; and (2) additional anatomic areas may be included in the CTV based on standard patterns of spread for the particular tumor type (e.g., nodal basins for head and neck cancer). In the end, the goal is to outline all areas that should receive the intended dose. In some situations, such as definitive prostate treatment and most SBRT, GTV = CTV.

Whereas GTV and CTV definitions are the job of the physician, definition of the planning target volume (PTV) is mainly the responsibility of the physicist and treatment planner because the goal of the PTV is to make sure that the CTV is adequately treated in the face of setup error, inter- and intrafraction motion, delineation/registration errors, and other errors in the planning and delivery process. The definition of the PTV should be done with as much information as possible because the region between the CTV and PTV contours is all “normal tissue” and increasing the PTV margin will cause more normal tissue to be irradiated. The physician is also involved in PTV definition by thoughtfully ordering appropriate patient or tumor immobilization for simulation and treatment. If a large PTV will not produce significant toxicity and the prescribed dose will be low, then immobilization can be basic. If, however, each additional millimeter of PTV margin will clinically make a difference, then strict immobilization must be ordered.

Often, the PTV is designed by simply defining an isotropic margin (e.g., 0.5 cm), and the CTV is expanded by this margin to create the PTV ( Fig. 21.4 ). This expansion should be performed in three dimensions because expansion of contours only in the axial plane will lead to PTVs that are not correct in the third dimension. If the uncertainties are not isotropic but are larger in one direction than in the others (e.g., as a result of respiration), then the margin to be applied should be anisotropic.

Fig. 21.4, Noncoplanar intensity-modulated radiotherapy plan for treating a brain tumor, with the ICRU-50 (International Commission on Radiological Units) target volumes shown: gross tumor volume (white) , clinical target volume (pink) , and planning target volume (yellow) .

There has been a great deal of work studying patient positioning, motion, and target volume delineation errors. Analysis of these issues has led to specific recommendations for the size of the margin between the CTV and the PTV. As described in the “Consideration of Setup Error and Patient Motion” section, one reasonable method for deciding the PTV-CTV margin has been determined to be (2.5 × Σ) + (0.7 × σ), where Σ is the standard deviation of the systematic error and σ is the standard deviation of the random errors for the population of patients treated in that particular site. To apply this formula, it is important to have measured, for your institution and each clinical site, the two different standard deviations. As can be seen from the formula, the systematic errors in the process, such as incorrect contouring or use of a nonrepresentative CT scan for target delineation, are much more important issues than random day-to-day setup errors.

Further discussion of other types of target volumes, including the internal target volume (ITV), is included later in the “Motion, Setup, and Four-Dimensional Anatomy” section.

For an individual patient, there can be multiple sets of GTVs, CTVs, and PTVs because there are often a CTV and PTV that correspond to each individual GTV. In the head and neck, where often a number of different nodal CTVs need to be treated, it can be important to develop an organized and clear naming convention for the various CTVs and PTVs. The AAPM Task Group 263 report on nomenclature can help with this issue.

Normal Tissues

Definition of normal tissues is also a critical task for conformal therapy because identifying the critical tissues will allow the treatment planner to avoid or at least minimize delivery of dose to those normal tissues. The planning tools used to avoid these structures can be simple graphic tools such as the BEV display, which allows the planner to choose beam angles and shape the radiation fields to avoid important structures, or it may involve detailed dosimetric and DVH analysis, as is often the case for IMRT planning.

To perform DVH or other dosimetric analysis, it is important that each organ to be analyzed be contoured completely because most current DVH data are characterized with respect to the whole organ's volume (either absolute volume or as a percentage of the whole organ). This has several implications:

1.
The CT scans or other studies for a given site must image all of the relevant normal tissues completely as well as the tumor. For example, a lung tumor CT scan should always include images from the neck down to below the bottom of the diaphragm, so that the complete volume of the relevant normal tissues can be identified (e.g., lung, heart, esophagus), particularly if the dose-volume limits that will be used for planning and plan evaluation are based on percent of the total organ.
2.
Contouring of normal structures must be done consistently if the DVH information is to be useful. Whole organs should always be contoured. For tubular structures (e.g., spinal cord, rectum, and esophagus) that can often extend out of the region of the tumor, it is important to have a defined protocol for how the structure will be defined so that the superior and inferior extents of the contours for the structure are specified (e.g., the rectum during prostate planning and the esophagus during lung planning). This is particularly important if relative volumes (e.g., <50% receiving >50 Gy) or mean doses of these normal structures will be used for plan construction and evaluation.

Multiple Imaging Modalities: Dataset Registration and Fusion

Although typically CT scans are the primary imaging modality used for radiotherapy planning, information from other types of imaging, particularly MRI and PET, can be useful for identifying disease or better identifying functional or anatomic areas that should be spared. Target volumes and normal structures can be identified on these additional imaging datasets and that information can be incorporated into the treatment plan along with the contours and data from the CT scans. As described before, a CT scan set is typically taken to be the geometric basis for the treatment planning because the CT data are of high resolution, are geometrically accurate, describe the electron density information needed for inhomogeneity corrections, and are quickly obtained.

Several issues need to be solved to make quantitative use of the additional imaging information.

For each imaging procedure, the patient would ideally be positioned within the imaging device using the same position (including breathing state) and immobilization and localization devices as for treatment. However, there are often positioning differences that must be taken into account.
Even if the patient positioning is perfect, the coordinate system used in the new imaging device is usually different than the coordinate system used by the original CT scan. Therefore, a geometric registration of the new imaging information must be performed to align the new images with the base imaging information. Otherwise, with different coordinates, one does not know how to map pixels in the new image dataset to the coordinates of the original image set.
For many imaging modalities—for example, MRI—various kinds of distortion are possible. If the new image dataset has geometric distortions, then these must be corrected (or at least accounted for) before the imaging information can be transferred into the base coordinate system for use in planning.
The resolution, slice thickness, and slice orientations of different imaging modalities can often be different than those of the original CT dataset; thus, any quantitative use of the new imaging data must also handle these kinds of differences.

To address these issues, the process of dataset (or image) registration is used to transform the coordinates for the various imaging datasets so that information can be passed from one image set into a coordinate system to be used for treatment planning. The registration process finds the geometric transform between the new dataset's coordinate system and the base coordinate system (typically, the CT scan). If one considers only rigid body registration, the transform can consist of x , y , and z translations, or both translations and rotations, and it can include scaling as well (although, typically, the scale of each dataset is known accurately and should not be modified).

Handling of deformable image registration is an important current research topic. Researchers are developing methods for mapping deformations from one system to another so that the deformations resulting from imaging or patient motion (e.g., respiration) can be taken into account. Many different mathematical methods have been employed, including thin plate splines, B-splines, demons, and others. However, the main current issue is that deformation is handled well for tissues that are imaged well, but there is no way to evaluate the quality of the deformation mapping for tissues that are not well imaged. Additionally, most current algorithms do not take into account anatomic constraints (elasticity of particular organs and subportions) and sliding organs (e.g., the diaphragm and lungs during respiration). Much work remains to be done here.

To determine the best registration transform, an optimization algorithm is applied to the problem. The optimization process consists of choosing the metric to be optimized (some metric that describes the quality of the registration) and choosing an optimization search algorithm that will perform the search over possible transforms so that the optimal one can be found. The metric can be something as simple as the sum of the squares of the distances between predicted and actual point locations, if it is possible to define point-based landmarks on both imaging studies; or it can be image-based metrics, such as the correlation between gray-scale values of two CT scans; or mutual information that can be used to register different image studies (e.g., CT and MRI or PET). This is a rapidly developing area of research.

No matter what kind of registration algorithm is used, it is necessary to verify the registration and then to use the data from the various imaging studies. Verification typically consists of image-based or structure-based comparisons between the two datasets, with the goal of confirming that known structures from the two imaging studies accurately line up ( Fig. 21.5 ). The quality of the registration depends on what parts of the images are most important clinically and must be reviewed by the planner and physician because, at this point, no quantitative measure accurately takes into account all of the clinical knowledge of the case. Once the registration is verified, then contours or 3D structure definitions from one dataset can be transferred into the base coordinate system for planning. This combination of data from multiple imaging sources is sometimes called image fusion .

Fig. 21.5, Split-screen comparisons of CT and MR image data to confirm the accuracy of the geometric registration of CT and MR scans of the liver. CT, Computed tomography; MR, magnetic resonance.

Motion, Setup, and Four-Dimensional Anatomy

Until the late 2000s, there had been little consideration of the fact that real patients breathe, move, are different from day to day, and change over time. Also, it was difficult to take such motion and localization differences into account within treatment planning, with the exception of defining appropriate PTV margins for the tumor. Fast helical CT scans, fast MRI, 4D CT, and 4D CBCT imaging using the treatment machine now provide detailed anatomic data as a function of time. These data have clearly demonstrated that a static anatomic description of the patient is not always appropriate and that treatment delivery schemes must also consider setup and motion effects if we are to achieve the optimal delivery of dose to the patient.

Several methods to handle motion and setup effects are in use or being investigated:

As described in the “Target Volume Definition and Margins” section, the standard way to handle motion and setup error is to determine the appropriate margin and then to expand the CTV by that margin to make a PTV that is the target for planning. If done correctly, the PTV ensures that the high-dose region always encompasses the CTV, even as the CTV moves around because of motion or setup error. The price, however, is that the larger the margin is, the more normal tissue is irradiated. Therefore, if the motion and setup error are controlled, this margin may be decreased, reducing the amount of normal tissue irradiated.
One popular intermediate strategy for handling motion issues is to define the target volume extent at both ends of the respiration cycle. By combining those volumes, an ITV is created that shows all of the locations that the target volume will move to during respiration; thus, it shows the envelope of volumes to be treated if the main goal is target coverage. As with the PTV margin, this ITV ensures coverage of the target but increases the amount of normal tissue that is irradiated. By ICRU definition, the ITV also includes the CTV, but in the era of 4DCT, “ITV” often is used interchangeably with the concept of the motion envelope of the GTV(technically, the iGTV), and additional CTV margins are sometimes then added to this structure after its creation. More recently, various clinical trial groups and physics associations have introduced the nomenclature iGTV and iCTV in order to avoid any confusion over the more generic term, ITV. One problem with using only the end-inspiratory and end-expiratory phases of the breathing cycle is that this does not capture the hysteresis of respiratory motion, typically an anterior motion in the middle of the cycle.
Stereotactic treatments and treatments using daily setup correction (IGRT) try to significantly reduce the daily setup variations by carefully reproducing the position of the patient each day.
Using imaging with 4D CT, respiratory-correlated CBCT, and other such 4D methods, it is now possible to visualize the motions of the tumor (and normal tissue) with the CT data and to create structure outlines that better approximate the clinical targets and structures to be avoided. This technology does capture respiratory hysteresis.
With use of ABC or patient-controlled respiratory methods, including voluntary breath hold with and without the aid of an external device, it is possible to control the breathing of the patient with the goal of turning on the treatment beam only when the patient is in the correct breathing state. This method can help minimize respiration motion and allow smaller PTV margins as well as potentially push organs at risk (OARs) away from targets (e.g., deep-inspiration breath hold (DIBH) for breast treatment). Similarly, abdominal compression devices can be used to dampen respiratory motion and reduce the iGTV and iCTV sizes required for treatment.
It is also possible to gate the radiation beam, turning it on only when the target structure is in the correct location, based either on external or internal markers or fiducials, although caution must be applied in choosing an appropriate surrogate for tumor position due to poor correlation of internal and external anatomic structures in different phases of the respiratory cycle and the decreasing correlation of tumor and surrogate position (including implanted fiducials) with increasing distance between them.
Various radiographic and radiofrequency transponder markers are also used to help identify the position of the target. Transponders that identify their position 10 times per second allow real-time tracking of the target and can help to dramatically reduce the PTV margin needed for treatment. This system is currently approved for use in the prostate, lung, and abdomen. Systems are also in use for surface setups, such as for breast radiotherapy setup guidance and treatment position monitoring.
This is a rapidly changing area, with continuing technical progress expected.

Use of Functional Imaging Modalities

Standard CT and MRI techniques basically provide anatomic information, although some simple physiologic information is obtained from MR. However, PET, SPECT, and various fMRI imaging sequences can all lead to much more detailed knowledge of the physiologic and functional state of various tissues or organs. The details of these many different types of studies are well beyond the scope of this chapter. However, several features of any such use are similar: the patient must be positioned as close to the treatment position as possible, the imaging study must be carefully registered geometrically with the rest of the treatment planning imaging information, and the actual meaning of the imaging information should be known. Although many different imaging scans show some response in the image, it takes detailed clinical knowledge and trials to clearly define the clinical meaning of any imaging result, a crucial step before the imaging information is used clinically. The development and study of these functional and physiologic imaging markers (imaging biomarkers) is a critical and fast-developing field.

Plan and Beam Definition

After the anatomic model of the patient has been established, the next major step in the planning process is to use the planning system to create a set of beams to be used for planning. This collection of beams, usually known as a “plan,” can be created using standard protocols (“treat all prostates with a six-field conformal plan”) or designed based on the specific anatomy of the case. Basic decisions on beam technique are often made early in the planning process, using experience or site-specific protocols. Typically, these decisions include picking the energy and number of beams, the basic orientations for the beams, and the type of beam shaping or intensity modulation to be used. The beam arrangements for different types of plans will vary between sites as well as between institutions: some facilities will have a standard set of beams and beam angles that are used for each type of plan just as they standardize nomenclature for target naming. This can be beneficial and improve the efficiency of the workflow. However, treatment planners should also be given the freedom to use beam arrangements that help achieve the optimal plan for each case.

Beam Technique (Energy, Direction, and Type)

The first elements to be decided as a treatment plan is generated are the number of beams to be used, their energy (and modality), and their direction. These choices are all interrelated; typically, this decision is based on standard experience or protocols as well as the available treatment machine capabilities. Although it is hard to summarize all of the useful ways to make this decision, there are a few standard rules that apply to most conformal planning.

Simple nonconformal planning often used opposed beams (anterioposterior-posteroanterior [AP-PA] and opposed laterals or obliques) to create a relatively uniform region of high dosage extending through the patient. For conformal planning, typically, pairs of opposed beams should be avoided in order to conform the dose to the target. For IMRT plans, this is even more important because opposed beams can cause degenerate solutions for the optimization search, leading to the optimization bouncing between two fairly equivalent solutions for opposing beamlets.
Virtually all conformal plans have at least three beam directions, and seven, nine, or many more beam directions are sometimes used (e.g., conformal stereotactic radiosurgery). Plans with more than seven beam directions are relatively uncommon when forward conformal planning is used, due mainly to the complexity involved in designing and determining the correct beam orientations, shapes, and weights interactively when there are so many different beams to consider.
Typically, lower energies are used for relatively superficial targets or for beams that traverse less normal tissue on the way to the target, whereas beams that must penetrate a greater volume of tissue are typically of higher energy. For example, whole-brain cases are most often planned with 6 MV while pelvic plans most often use 10 MV or higher, although good multifield conformal plans can also be created with 6 MV even for the pelvis. Changing energies for individual beams often makes a relatively small difference in the dose distribution achieved for a conformal plan; this possible improvement should be balanced against the additional complexity of the plan if multiple photon energies are used.
For sites in which an attempt is being made to ensure that the dose in the buildup region is relatively high, lower energies are usually used because there is less skin sparing for these beams. Field size is also important for skin sparing: larger fields have less skin sparing than small fields.
The most conformal plans can be achieved by ensuring that there are beams arranged around the target in all three dimensions so that the plan includes some noncoplanar beams. If nonaxial beams are used, special care should be taken to avoid collisions with the patient or the treatment machine: paying close attention to both the patient anatomy and the physical machine characteristics is a must.
There is still a great deal of debate over the best energy beams to use for plans that involve large inhomogeneities such as the lung, although generally lower-energy beams are often preferred owing to the difficulty associated with inhomogeneity-corrected dose calculations for small high-energy fields, especially. Many current clinical trials suggest the use of low energy (e.g., 6 MV) for beams that do not need to travel more than 10 cm through solid tissue.
In creating more complex beam arrangements that have many degrees of freedom available to be optimized (e.g., static IMRT), it is possible to achieve highly conformal plans. Virtually all IMRT plans have at least three different beams, and many beam directions are sometimes used. In much of the early IMRT literature, it was common to find plans with seven or nine axial beams, with beam directions evenly distributed about the patient. This number of beams allowed enough flexibility for the optimization to achieve an adequate plan, even if the beam directions were not tuned with respect to the anatomy. However, as IMRT planning has progressed, the use of nonaxial and geometrically optimized beam directions has become more popular. Contrary to some early IMRT literature, it is still useful to use beams from outside the axial plane because they help to improve conformality. However, with IMRT, it is often obvious that the nonaxial beams are useful only when considering low-priority normal tissues because the IMRT optimization can typically achieve all of the higher-priority goals without use of nonaxial beams. One feature of most IMRT plans still remains the same: typically, the use of opposed beams is avoided because opposing beams tend to lead to degenerate solutions that can cause difficulties for some optimization search algorithms, preventing them from reaching the optimal solution. Additionally, when targets are well lateralized, it is generally preferred to use lateralized beams or restricted partial arcs to avoid entrance in the contralateral part of the body.
Another type of IMRT, VMAT, uses one or more arc beams along with intensity modulation during the arc delivery. VMAT plans often consist of two or three arcs to allow the optimization process enough degrees of freedom to achieve the desired plan. The most standard VMAT delivery method consists of axial arcs, but now the use of arcs outside the axial plane are sometimes used to improve conformality. Typically, the collimator is rotated from the default position differently for each arc to ensure that there are independent degrees of freedom for the optimization process and to make sure that the tongue-and-groove issues associated with the MLC orientation do not line up consistently in the different arcs.
Often, particularly for central targets, complete arcs are used for VMAT (although technically commonly limited to 358 or 359 degrees to avoid identical start/stop gantry angles), while lateralized targets may be best treated with partial arcs. The number of arcs used is dependent on the planner, protocol, and complexity of the case. Two arcs are most common, while the most complex cases with forced heterogeneity and challenging geometry may require three arcs, and a simple small, round tumor may be well treated with a single arc. It is much easier to control low-dose spillage (to normal tissues around the target) with a step-and-shoot IMRT plan than a VMAT plan, mainly due to the fact that VMAT plans are delivering dose from all angles as the gantry rotates around the patient. On the other hand, VMAT reduces the jagged edges often seen with a step-and-shoot IMRT plan.
There has been much discussion suggesting that all IMRT plans can be accomplished with 6 MV photons and that higher energies are not necessary, but there is little hard evidence to demonstrate that such a conclusion is correct. In addition, it is often said that IMRT plans can be achieved only with axial beams, but the use of noncoplanar beam arrangements will still result in at least slightly better plans (typically reducing somewhat the normal tissue dose for the same target conformality).
Even when using IMRT, VMAT, or conformal arc therapy, beam selection is important because it can limit the degree of normal tissue sparing or target coverage that can be achieved. For example, if beams are placed so that they shoot through the kidneys, no matter what IMRT cost function is used, kidney dose will be higher than if the beams are aimed outside of the kidneys. For arc therapy or VMAT, if the arc is defined to avoid a particularly crucial organ, sparing will be improved.

Beam Shaping With Blocks and Multileaf Collimator

Although beam directions are important, shaping the radiation field to conform to the shape of the target volume is one of the crucial and defining concepts for 3D CRT. The shaping can be accomplished equally well by focused blocks or with an MLC, as illustrated in Fig. 21.6 . The conformal shaping of focused blocks is, in fact, “more conformal” than the jagged shape created by an MLC, although the MLC has a number of other advantages that have led to its popularity.

Fig. 21.6, Shaped blocks and multileaf collimators can be used to shape the beam conformally.

The routine use of conformally shaped fields designed during treatment planning depends in large part on the availability of the BEV display in the planning system because this view of the target shows the projection of the shape of the targets from the point of view of the radiation beam, which is what is needed to design field shaping. The simplest method used to conformally shape the fields (with either blocks or MLC) is to create a uniform geometric margin around the projection of the targets in the BEV and to set the shape to that margin, as shown in Fig. 21.7 . This method, the basis of the simplest type of conformal therapy, is sometimes called geometric conformation , or BEV targeting . Shaping a block to a given contour is easy, but with an MLC it is more complicated. The most commonly used method for an MLC is the so-called equal area method (see Fig. 21.7 ).

Fig. 21.7, When using geometric conformal shaping for a multileaf collimator (MLC), the most common method is to create a contour at the desired margin (red contour) and then push the MLC leaves up to that contour so that the areas overlapped and underlapped by each leaf are equal.

Using a uniform geometric margin for field shaping does not lead to the most conformal dose distribution. To truly conform the dose distribution to the target, one must optimize the shaping of each of the beams so that the dose distribution is conformal. Fig. 21.8 demonstrates the types of differences that occur when beam shapes are designed with a uniform margin and when the shapes are optimized to conform the dose to the target. Beam directions, the penumbra, and how the beams cross-fire on the target affect the margins required for individual beam shapes.

Fig. 21.8, Beam's-eye view of conformal fields: uniform geometric margin (A) and conformal dose distribution (B). Using a uniform geometric margin typically does not achieve a dose distribution that is as conformal as possible. For this three-field pancreas treatment plan, the beam on the right had its shape optimized to make the dose distribution more conformal to the target.

Collimator angle is one more factor that can directly affect the conformality of a plan when an MLC is used. The leaves from an MLC move in only one direction; thus, to minimize the “stair-step” or jagged edges caused by MLC leaves when shaped to an angled contour, one may use collimator rotation so that the MLC leaves best fit the shape of the target or normal tissue. Minimizing the jagged MLC edges decreases the penumbra between target and normal tissues. For example, to make a sharp dose gradient between the prostate and rectum, rotating the collimator to parallel the edge of the rectum can help make the edge sharper. As a general rule, rotating the collimator so that it is parallel to the long axis of the target will also help create a more uniform dose distribution.

It is possible to create intensity-modulated beams using a limited number of MLC-shaped “segments” all from the same gantry angle. This “segmental” (or “field-in-field”) IMRT can be created using the normal interactive planning paradigm (“forward planning”) or the limited number of segments can be created by an inverse planning paradigm (see, e.g., the work by Shepard et al. regarding direct aperture optimization ). The use of a few segments to improve target homogeneity (e.g., in the treatment of breast cancer with tangential fields) is a logical extension of the concept of wedged tangents when 3D planning is available.

As an example of field-in-field planning, consider a pair of tangential breast fields. With simple tangent fields, a hot spot will be created at the thin part of the breast. Cooling off the hot spot was traditionally achieved with wedges, but the forward planned or “field-in-field” plan can create a more homogenous dose distribution because the MLC apertures can be shaped to the contour of the breast and the dose distribution. To address the hot spot in a tangential breast plan, one can copy the field and create “subfields” that block out the hot spot with the MLC. As illustrated in Fig. 21.9 , blocking out the 110% and 105% isodose surfaces (IDS), for example, can be done with low-weight subfields to generate a reasonably uniform dose distribution. This technique can be used on any clinical site, though it is most commonly used for breast cases.

Fig. 21.9, Field-in-field segments in a tangential breast field. (A) Open tangent field. (B) Segment blocking out the 105% hot spot. (C) Segment blocking out the 110% hot spot.

Other Beam Technique Decisions

There are numerous other decisions to be made when creating the plan.

One must choose the intensity of each beam, typically called the beam weight . How this decision is made depends on the specific beam normalization methods used within the planning system. Because the beam weight, or intensity, is directly related to the number of monitor units (MUs) that will be delivered from each beam, it is essential that translation of beam weights into MUs be understood and carefully handled because errors in this step deliver the wrong dose to the entire field.
For certain beam combinations, use of wedges in the field can help achieve a more uniform dose in the target volume. Standard two-dimensional (2D) planning makes routine use of pairs of fields known as “wedged pairs,” in which the wedges help compensate for the fields entering the patient at 90 degrees (or some other angle) to each other. For conformal planning in three dimensions, there is a broader range of combinations of fields for which wedges are useful. For example, a 3D cone has some number of fields placed around a cone, and each of the fields usually requires a wedge to keep the target dose uniformity. This beam distribution is the 3D analog of the wedged pair. Multiple segment (field-in-field) planning can be used to replace or improve on the use of wedges for target uniformity.
When the target is near the surface of the patient, it is often difficult to achieve the desired dose coverage of the target because of low doses in the entrance region of megavoltage photon beams. In this case, some amount of bolus can be added to the skin of the patient, for one or more beams, so that the photon beams can transit more material, therefore increasing the dose at the actual skin surface. This bolus description can be designed using the planning system as long as care is taken in creating the physical bolus so that it actually matches the description that was put into the planning system.
Most conformal treatment plans are isocentric, with the isocenter of the machine placed within the center of the target and the beams rotating around the isocenter in the target. Other arrangements are possible, depending on the geometry of the normal anatomy and targets. For some situations, it is also possible to create pseudo-isocentric beams, which use 3D table motions to move the patient away from the head of the machine along the beam central axis so that the plan is apparently isocentric but with a larger source-to-isocenter distance than the physical machine isocentric distance. This technique can be used to create a larger range of table and gantry angles that can be used for treatment without the machine's head getting too close to the patient.

Intensity-Modulated Radiotherapy

Soon after conformal therapy began to be used clinically in the late 1980s, Brahme, Bortfeld et al., and others introduced the idea of modulating the intensity across each radiation beam, assisted by computer-based optimization algorithms to help determine the intensities required of the different parts of the beam. This combination of beams with modulated intensities and the use of optimization (inverse planning) is commonly called intensity-modulated radiotherapy (IMRT) and is now a commonly used method for the creation of highly conformal treatment.

Intensity-modulated fields can be achieved in a number of different ways. There is a continuum of situations ranging from a flat field to multiple-shaped segments to a beamlet-type description (referring to dividing the beam cross-section into numerous “beamlets,” each of which have an independent intensity or fluence). The intensity modulation is created by either a series of SMLC segments or a dynamic DMLC sequence. In the past decade, VMAT has been an increasingly used method in which the gantry is in motion during a DMLC sequence with variable dose rate. VMAT can typically increase delivery efficiency with similar plan quality to static field IMRT. However, both methods have advantages or disadvantages for different patient scenarios.

For plan optimization strategies, there is a similar range from simple (forward) iterative planning to optimized (inverse) planning that is driven completely by a mathematical cost function. Typically, the most complex intensity distributions (IMRT fluence and VMAT) are generated with inverse planning, but it is also possible to perform optimization of static segments for conformal therapy.

Dose Calculations

Once the initial treatment plan is designed, the next step is typically to perform a dose calculation so that the planner and physician can evaluate the dose distribution expected from the plan. Currently, because biological effects are not well documented and understood in general, the physical dose distribution is the main characteristic that is used (1) to choose between plans, (2) to choose what dose to deliver to the patient, and (3) to evaluate the quality of various plans proposed by the planner.

You're Reading a Preview

Become a Clinical Tree membership for Full access and enjoy Unlimited articles

Become membership

If you are a member. Log in here