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Special Procedures
24.1
Introduction
Special procedures are defined by the following characteristics:
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Unique beam configuration, accessories, or radiation delivery device settings compared to other treatments such as 3D conformal radiation therapy (3D CRT) or intensity-modulated radiation therapy (IMRT)
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Relative number of patients treated is small
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Not widely offered in all clinics because of the relative effort to implement a special procedure compared to the number of patients seen
Some special procedures such as total body irradiation (TBI) and total skin electron treatments (TSET) have been staple tools of clinical treatment for many years, while others such as radioactive microspheres for liver metastases are relatively new. New device or drug developments can quickly change the number of patients treated with a special procedure; for example, the development of medicated stents eliminated the use of intravenous brachytherapy within a short time. Some special procedures are still in the process of settling with a medical specialty; for example, radioactive microsphere treatments are performed by radiation oncologists, nuclear medicine physicians, and interventional radiologists.
Also considered in this chapter is the issue of pregnant patients. This is a “special procedure” in the sense of assessing pregnancy status and designing specialized treatment if it is decided to proceed with treatment while the patient is pregnant.
24.2
Total Body Irradiation
The purpose of TBI is to prepare a patient for bone marrow (hematopoeitic stem cell) transplant, either allogeneic or from a donor. This transplant is employed in the treatment of leukemia, lymphoma, or occasionally for nonmalignant disease such as common variable immunodeficiency and aplastic anemia. High doses of TBI can eliminate residual cancer cells. The irradiation in combination with chemotherapeutic agents acts to one of two purposes: to ablate the patient's hematopoietic stem cells in the bone marrow (myeloablative TBI) or to suppress the immune system of the patient prior to transplant (nonablative). With improvements in chemotherapeutic agents, the number of TBI patients has been decreasing over recent years, although it remains a key component of some transplant protocols.
Nonablative TBI regimens typically use a dose of 200 cGy × 1 fraction, whereas the myeloablative TBI regimens typically use 1200 to 1550 cGy delivered over 3 to 5 days, typically BID. Various boosts (such as testicular boosts) may also be employed in addition to TBI. The common characteristic through most protocols is the use of a low dose rate (typically 5 to 15 cGy/min at midplane) to minimize toxicity such as pneumonitis. The extended treatment source-to-surface distance (SSD) usually is sufficient to bring the dose rate into the range of preset linear accelerator (linac) dose rates. For some pediatric setups, a shorter SSD may require special tuning of the linac to achieve the required dose rate. TBI treatments rely not only on accurate execution of the treatment plan, but also on precise timing of the treatment delivery. A second linac on which TBI treatments can be delivered should be available as backup in case of equipment failure. It is often not an option to delay the TBI treatment of a patient, because this irradiation is carefully timed in advance with the conditioning regimen and the transplant.
Because TBI patients are treated on a wide range of protocols in combination with other therapies, the dose fractionation scheme is often set by the choice of treatment protocol in oncology and carefully coordinated with the stem cell transplant (autologous or allogeneic). Incorrect dose delivery can lead to severe or even fatal toxicity. Good communication between oncology and Radiation Oncology is therefore essential to ensure the correct protocol is selected for treatment planning and treatment. Best practice is to request a copy of the treatment protocol for a dosimetry and physics check to verify the correct dose fractionation scheme was planned. Expected early side effects of treatment include the typical symptoms of whole body irradiation: nausea, fatigue, and/or headache.
The American College of Radiology/American Society of Therapeutic Radiation Oncology (ACR/ASTRO) practice guideline for the performance of total body irradiation contains detailed descriptions of the qualifications of the patient care team (physicians, medical physicists, radiation therapists, nurses, and dosimetrists). This practice guideline emphasizes the importance of a well-qualified team of physician subspecialties and caregivers specifically trained for the administration of this high risk procedure. Incorrectly administered or medically supported TBI carries severe toxicity risks including death of the patient. In addition to being trained in their respective skills, all caregivers need to work as a team and verify that every caregiver has a thorough understanding of the procedure and his or her role in it. End-to-end dry runs of all protocols used in clinical practice are a highly effective training tool and can also be used to verify that policy and procedure guidelines are complete and accurate.
American Association of Physicists in Medicine Task Group (AAPM TG)-29, The Physical Aspects of Total and Half Body Irradiation, was published in 1986 at a time when the results of large clinical trials incorporating TBI were not yet available. Nevertheless, much of the discussion on dose accuracy, clinical reasoning, and the basic principles of irradiation design are unchanged. The one exception is the use of hemi-body irradiation for widespread metastatic disease; better chemotherapy agents, earlier cancer detection, and new approaches to oligometastatic disease treatments have rendered this technique extinct except for limited applications in veterinary Radiation Oncology.
Patient Setup for TBI
The most commonly used setup for adults is in a standing position on a small pedestal. This position provides many advantages: easy positioning of beam spoiler and lung blocks, less variation in separation compared with the seated position, and good spacing away from walls and the floor to reduce backscatter. The major disadvantage is that it requires the patient to stand still for an extended time period of 20 min to 30 min or longer, which is very challenging for many TBI patients. TBI stands are often custom-built, and different clinics have developed adaptations to provide patient support:
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A custom-mounted bike saddle of adjustable height can support some of the patient's weight without interfering with the beam excessively.
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Railings or bars for the hands can assist the patient with keeping balance.
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To minimize the fall risk, the patient could wear a full-body climbing harness secured by a belay anchor in the ceiling. Other solutions include Velcro straps or lateral support bars.
The beam arrangement for a standing setup is at an extended SSD of approximately 400 cm. With an open 40 cm × 40 cm field and a collimator rotation of 45°, this results in a maximum field dimension of 225 cm. For patients who cannot stand for the duration of the treatment, an alternative is to be placed in a sitting or supine position on a gurney. Instead of anterior-posterior (AP/PA), the fields are treated from the lateral direction. Because there is a significant change in separation from the shoulders to the head/neck area, tissue compensators are almost always used. Lung blocks are not possible with this setup, although the arms provide some natural blocking. Some centers also use a decubitus lying-down setup for patients who cannot stand. This arrangement allows AP/PA treatment and can accommodate lung blocks, though positioning may be less stable.
Small children up to about 3 years of age can be placed in a frog-leg position on a special bed or vac-lock bag on the floor. The main consideration is whether they will fit into the field size projected onto the floor. The beam spoiler is mounted above the patient, and the lung blocks can be placed directly on the beam spoiler. Taller children can be on the floor and treated with two fields (gap on the thigh with feathering) or treated on a gurney. For older children, the choice between gurney and a standing position has to be made based on the maturity and cooperation of the child. Anesthesia is an option for immobilization, but a tablet computer with a game or movie in the child's hands can be a surprisingly effective immobilization device with considerably less associated morbidity than with anesthesia.
Some clinics have developed technologies to treat the patient on a moving couch or on a fixed couch with a radiation source sweeping over the patient. These techniques are exceedingly rare and will therefore not be discussed here.
Beam Spoilers, Tissue Compensators, and Lung Blocks
A beam spoiler is always used in TBI treatments for the purpose of increasing lower energy components in the beam to avoid skin sparing, which is not desirable in TBI protocols. The spoiler is typically made of clear plastic such as acrylic or Plexiglas. The spoiler is placed about 20 to 30 cm away from the patient.
In myeloablative TBI regimens, it is typically necessary to block the lungs to reduce the incidence of pneumonitis; this is the case for most treatments with the exception of single-fraction regimens. Lung blocks can be mounted either on the beam spoiler itself or on a special accessory tray between beam spoiler and patient. The purpose of the lung blocks, contrary to their name, is not to block dose to the lung completely (lungs may receive ≈70% of the prescription dose). They serve instead as tissue compensators to avoid overdose in an area of the body where low density lung tissue decreases the effective thickness significantly compared to the separation at the umbilicus. The position of the lung blocks is verified using a megavoltage (MV) radiograph and film during simulation and treatment. The position of the lung block shadows on the patient should be marked as reference points.
Depending on the protocol, tissue compensators may be required to increase homogeneity of delivered dose across the patient. A typical protocol goal is ±10% dose homogeneity to all tissues. A common method for compensator construction is the use of lead strips of about 2 mm thickness layered onto an accessory tray that is then mounted on the linac accessory tray.
Commissioning Measurements and in-Vivo Dosimetry
AAPM TG-29 Section 3 describes in detail the measurements required to commission a TBI procedure using the reference dosimetry methods described in AAPM TG-21, A Protocol for the Determination of Absorbed Dose from High-Energy Photon and Electron Beams; these should be replaced by the reference dosimetry methods of AAPM TG-51, Protocol for Clinical Reference Dosimetry of High-Energy Photon and Electron Beams. Although the use of solid water phantoms for reference dosimetry is acceptable, it is best practice to use a large water phantom for annual calibrations to perform these measurements. Commissioning data (PDDs, output factors, beam profiles) should be gathered for the clinical setup used (i.e., at extended distance with the beam spoiler and compensator tray in place).
During commissioning of the TBI technique, the in-vivo dosimetry system should be placed on an anthropomorphic phantom in treatment setup to verify the dose calculation and dose inhomogeneity across the treatment field. Radiochromic film should be placed between axial slices in the phantom at the same levels as the in-vivo dosimeters to assess dose inhomogeneity as a function of separation. AAPM TG-29 Figure 2 shows the ratio of Dose max /Dose midline as a function of patient thickness and beam energy. As the patient thickness increases, the ratio of peak to midline dose diverges with the lower energy beam ratio rising more quickly. For most patients, the choice of beam energy of 6 MV or 10 MV is appropriate; obese patients may require higher beam energies of 10 MV or 15 MV to achieve the ±10% dose homogeneity to all tissues. Therefore, all photon energies in this range should be commissioned for TBI.
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