Imaging for Radiotherapy Planning

External Beam Radiotherapy

External beam radiotherapy constitutes the mainstay of radiation treatment. It uses a radiation beam that originates at a distance from the patient and is directed towards a defined treatment volume. Approximately 85% of all therapeutic radiation exposures are delivered using external beam techniques. Various types and energies of radiation can be delivered in this way, including electromagnetic radiation such as x-rays and γ-rays, or particles such as electrons and protons. Higher energies of radiation penetrate deeper into body tissues. As a result, low-energy x-rays (60 to 300 keV) are reserved for the treatment of skin cancers and superficial subcutaneous tumours. Most external beam radiotherapy treatments use megavoltage x-rays or electrons (6 to 18 MeV) generated by a linear accelerator, which remains the workhorse of a modern radiotherapy department ( Fig. 67.2 ).

Conventional External Beam Radiotherapy

Conventional radiotherapy refers to techniques in which the treatment volume is defined by simple geometric parameters. In general, no attempt is made to delineate the tumour outline or to shape the radiation dose distribution to conform to the tumour volume. This is commonly practised for palliative treatments where long-term normal tissue toxicity is less relevant. The irradiated volume can be defined clinically or fluoroscopically, but more often CT simulation is used ( Fig. 67.3 ). The radiation fields tend to run parallel to each other, creating a box-like treatment volume.

Three-Dimensional Conformal Radiotherapy

The incorporation of axial imaging data allows 3D reconstruction of the tumour and surrounding organs. This provides more accurate localisation of the target volume and more information regarding the amount of normal tissue that will be irradiated. The radiotherapy-planning computer software uses the attenuation coefficient information (Hounsfield units [HU]) derived from the CT image on a voxel-by-voxel basis to predict the attenuation of each therapeutic radiation beam as it passes through the body. As a result, the number and profile of the radiation beams can be orientated and shaped to fit the profile of the target from a beam's eye–view using a multi-leaf collimator ( Fig. 67.4 ). The resulting radiotherapy plan can be displayed as a colour map of radiation dose overlaid onto the anatomical CT images so that the radiation oncologist can determine whether the tumour volume will receive sufficient irradiation with acceptable normal tissue dose sparing ( Fig. 67.5 ). By reducing the irradiated volume and the dose to the sensitive surrounding normal tissues, this technique facilitates the delivery of higher tumour radiation doses than would be achievable using conventional techniques.

Intensity-Modulated Radiotherapy

Intensity-modulated radiotherapy (IMRT) represents a further step in the development of high-precision radiotherapy delivery. The term refers to a variety of techniques in which the radiation beams are not only shaped and orientated to conform to the tumour volume but also the intensity of radiation is modulated across each treatment beam. This technique can produce dose distributions that conform highly to complex shapes, including treatment volumes that wrap around sensitive normal structures such as the spinal cord ( Fig. 67.6 ), enabling high-dose delivery to the tumour volume whilst sparing the normal structures.

IMRT can be achieved using a number of different technologies. Most commonly, several static radiation fields in the same plane of orientation are used, similar to the situation for 3D conformal radiotherapy but with varying dose flux across the profile of the beam, which is achieved by moving the leaves of the multi-leaf collimator across the beam at varying rates. Alternatively, arc therapies use a number of non-coplanar beam arcs in which the radiation is delivered using multiple ‘stop and shoot’ beams or as a continuously moving field that varies in intensity throughout rotation. Tomotherapy is another technique for achieving IMRT in which a megavoltage x-ray source is mounted in a similar fashion to a CT x-ray source. The treatment volume is irradiated using the machine's continuously rotating beam that is modulated in intensity whilst the patient moves through the gantry bore. Image guidance is critical when such complex highly conformal techniques are used. Motion management using 4D planning and delivery is also incorporated, particularly when treating sites affected by cardiorespiratory motion, such as the lung and breast.

Stereotactic Body Radiotherapy

In stereotactic body radiotherapy (SBRT) the distribution of radiation beams is in three dimensions and not in two as in traditional radiotherapy. It may be delivered using a suitably modified standard linear accelerator or dedicated machines such as CyberKnife ( Fig. 67.7 ). It is used for intracranial tumours where precise targeting of dose may be critical and selected, small extracranial lesions such as tumours of the lung and prostate, or small solitary metastases. The precision of SBRT allows high doses to be delivered in a very limited number of fractions. This is sometimes referred to as ultra-hypofractionation in which doses per fraction can be as high as 20 Gy (conventional radiotherapy is delivered at 2 Gy per fraction).

Brachytherapy

Brachytherapy refers to a treatment in which a radioisotope is placed onto or inside the volume to be treated. One of the key features of brachytherapy is that the irradiation affects only a very localised area around the radiation sources because dose falls off rapidly, obeying the inverse square law. As long as the sources are placed precisely within the tumour, there is minimal exposure to radiation of healthy tissues further away from the sources. This allows very high doses to be administered to the target volume.

Whilst in the early days of brachytherapy live sources were placed manually, this is no longer acceptable. The exception is low–dose rate prostate brachytherapy with ¹²⁵ I seeds. Here the gamma energy is only 28 keV, and the dose outside the seed in which the isotope is encapsulated is very low. In other settings the usual source is iridium 192, which is used in an afterloader. This works on the principle that non-active applicators, typically metal or plastic hollow tubes, will be placed in the volume to be treated, ideally with real-time imaging to direct them accurately. Ultrasound is particularly valuable for this.

Radiation is then delivered in a protected room by remote activation of the afterloader connected to the applicators. Passage of the source along the applicator is controlled, and the rate of passage will define the amount of radiation delivered; typically the source will ‘dwell’ for a few seconds at 5-mm intervals within the tumour volume. In brachytherapy, patient setup and tumour motion are less relevant because the radiation sources move with the tumour and therefore retain their correct position, addressing one of the major challenges in accurate radiation delivery.

Dosimetry in brachytherapy is based on addition of the contributions from each source position within the volume. Accurate imaging of the applicators and reconstruction of the tumour are essential. Brachytherapy applicators are CT and MRI compatible. CT is often preferred to identify accurately the position of the applicators, whereas MRI will usually give superior definition of the tumour, especially in the pelvis, where prostate and gynaecological cancers are commonly treated with this technique ( Figs 67.8 and 67.9 ). Specific MRI sequences may be developed to enable certain applications (e.g. proton-rich sequences for identification of ¹²⁵ I seeds in the prostate).

Particle Therapy

The most common particles used in radiotherapy are electrons. These are produced in the linear accelerator. The target, which produces x-rays when bombarded with electrons, is removed so that a beam of high-energy electrons is produced instead. Electron beams have a specific range, and the effective range is approximately one-third of the accelerating energy: for example, for a 6 MeV electron beam the effective range is approximately 2 cm. The advantage of electrons is that they have a defined range with minimal exit dose beyond that point. Therefore they are commonly used to treat skin tumours or other superficial structures such as the ribs.

Protons have an increasing role in clinical radiotherapy, with many centres across the world being established. Protons penetrate deep into body tissues and deposit most of their energy in the last few millimetres of their range, the Bragg peak, with virtually no radiation passing beyond this distance. The position of radiation deposition in the body can therefore be defined by choosing an appropriate energy of the proton beam. In practice, a range of energies to produce the ‘spread out Bragg peak’ is used to treat a defined volume. In this way, tissues behind the target volume are spared radiation dose. This may be critical in certain sites: for example, inside the cranium or around the spinal cord. It also has major advantages in children, in whom sparing of growing tissues is an important component of minimising late effects.

There are only a few centres in the world that have the capability of treating patients with other types of particle, such as fast neutrons, or carbon ions. These techniques have physical advantages similar to those of protons, with sparing of tissues at depth, but may have additional biological benefits. Both neutrons and carbon ions cause dense ionisation with much greater transfer of energy along the radiation track. This results in more radiation damage and cell death. Furthermore, there is theoretical evidence that intensely ionising radiation of this sort can overcome the detrimental effects of tumour hypoxia, which is a major cause of treatment failure with standard radiotherapy.