8 2011
IMAGING & ONCOLOGY
factors. First, not running too many rooms off one accelerator. Second, on the equipment being able to deliver a high dose rate so that individual fields can be treated in a short time and, finally, on being able to switch the beam between rooms quickly.
Treatment rooms will generally be a mix of fixed beam rooms where the beam can be directed at the patient only from one or two directions, and gantry rooms where the beam can be rotated around the treatment couch. Gantries for proton radiotherapy need to incorporate large and powerful magnets to bend the high energy protons. The resulting gantry structure can be three stories high, taking up the floors below and above the treatment floor level. These major pieces of engineering increase the initial cost, leading centres to treat with fixed beams where possible. For simpler treatments, a fixed beam, in combination with a robotic couch, can be used to deliver proton radiotherapy.
A good example is prostate treatment, where two lateral beams can be delivered by a fixed beam. A variant on the fixed beam is the dual inclined beam, where two beams at different angles can provide greater flexibility.
In some centres more complex plans may contain beams that can be delivered by fixed beams and beams that can only be delivered by gantries. In this situation the patient will receive treatment in different treatment rooms on alternate days with not all beams treated daily. This is possible with proton treatments as each field can deliver a uniform dose to the whole tumour.
PASSIVE SCATTERING VERSUS SCANNING TECHNOLOGIES Nearly all of the treatments that have been delivered so far use a scattered proton beam to treat the patient. Passive scattering uses a range shifter wheel, or ridge filter, to modulate the energy of a narrow beam to create the spread out Bragg peak (SOBP) to produce a high dose to the target from a single beam direction. The narrow proton beam is scattered to produce a broad beam that can be used to target the complete treatment field in a similar fashion to a photon beam (Figure 2). The scattered beam then uses a custom collimator to define the field size and a compensator to alter depth of penetration across the beam aperture.
The compensator and collimator used for passive scattered proton radiotherapy are made individually for each patient and are unique for each treatment beam used in the proton treatment plan (Figure 3). Each collimator/compensator pair for every treatment beam needs to be manually inserted into the treatment nozzle prior to the irradiation of the patient. Although an effective solution, this can lead to manual handling and radiation protection issues for staff at the proton radiotherapy centre, as well as increasing the cost of proton treatments.
Figure 3. Individual collimator and compensator used in passive scattering proton systems.
Figure 2. A passive scattering treatment system. Compensator Patient Target
Scatterer
Modulator wheels or ridge filters
Scatterer (contoured)
Collimator
Extra dose
Figure 4. A scanning proton treatment system.
Trteatment volume Magnets
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