Currently, two proton-beam delivery methods are available: passive spreading, which includes single and dual scattering; and active spreading, commonly known as beam scanning. In passive spreading techniques, the proton beam is spread by placing scattering material into the path of the protons. A single scatterer broadens the beam sufficiently for treatments requiring small fields.
For larger fields, a second scatterer is needed to ensure a uniform dose profile. A combination of custom-made collimators and compensators conform the dose to the target volume. The spread out Bragg peak used for treatment is obtained via a set of range modulator wheels or ridge filters inside the nozzle of the delivery system (figure 1).
In scanning-beam techniques, magnets deflect and steer the proton beam. Under computer control, a narrow mono-energetic beam paints the treatment volume, voxel-by-voxel, in successive layers. The depth of penetration of the Bragg peak is adjusted by varying the energy of the beam before it enters the nozzle (figure 2).
Here, we take a look at how the two technologies compare in a clinical setting.
Beam modification
Scattering-based delivery techniques employ patient-specific beam-modifying devices to conform the dose to the treatment volume. These must be manufactured in-house or outsourced, increasing the cost of treatment and delaying its start until the devices are ready. The devices, which become radioactive from exposure to protons, also need to be stored for months after use, necessitating a dedicated storage area and increasing facility costs. In nearly all cases, proton-beam scanning does not require any collimators, compensators or other beam-modifying devices, making this technique a more environmentally friendly option.
Another factor to consider when using scatterers and beam-modifying devices is the generation of neutrons when the proton beam hits these structures. This secondary radiation increases the integral radiation dose to the patient. Any such radiation that does not directly contribute to destroying cancer cells is obviously undesirable.
With no need for scattering material, scanned-beam proton therapy naturally produces fewer neutrons, thus reducing the integral dose delivered to the patient. Limiting neutron production is particularly important when treating children, who have an increased risk of developing neutron-induced secondary cancers later in life.
Whenever compensators are employed, some unnecessary dose is deposited in tissues close to the treatment volume. This occurs because the spread of the Bragg peak is constant across the treated depth. As a result, some tumours close to sensitive critical structures may not be optimally treated with dual-scattered proton beams.
Multiple fields can be used to improve target conformality. But this may not be an ideal solution, as switching the complex set of compensators and apertures required increases both the overall treatment time and the possibility of patient movement. Proton-beam scanning delivers lower doses to healthy tissue than other proton delivery methods, making it more likely that the patient will tolerate the treatment.
When protons encounter scattering material, they lose energy, along with penetrating power. Thus beam scattering also reduces the maximum depth of the Bragg peak that can be obtained. For any given accelerator, scanning enables protons to penetrate deeper than scattering, allowing treatment of deeper-seated tumours.
IMPT
Previous studies have indicated that intensity-modulated proton therapy (IMPT) results in lower integral dose than intensity-modulated X-ray therapy (IMRT). The ability to vary the dose distribution throughout the treatment volume is limited with scattering techniques. While multiple fields can deposit dose from different directions, switching the necessary compensators and apertures adds to the overall treatment time.
Scanning, however, can be used to deliver true IMPT, as dose distributions can be varied on a voxel-by-voxel basis. By varying the proton beam intensity and/or the speed of the scan, dose can be painted non-uniformly on a field-by-field basis to yield an overall uniform target dose. The scanning technique lets clinicians tailor treatments to improve dose conformity, reduce integral dose, or both.
Motion sensitivity
Proton therapy poses the same requirements for accurate, repeatable patient positioning and set-up as does IMRT. In addition, low- and high-density structures moving in and out of the beam – due to patient motion or tumour shrinkage, for example – will alter the range of the Bragg peak. As such, it's essential that the target volume and surrounding structures are in their planned positions.
While this requirement holds for all proton therapy techniques, the enhanced ability of proton scanning to paint dose conformally increases the risk of target misses due to organ motion. This risk can be mitigated by image-guidance techniques or by using multiple re-paintings that compensate for organ motion by effectively smearing out the dose. Scattering, on the other hand, is more forgiving of tumour and organ motion, because of the smearing effect of the broadened beam.
Finally, there's the matter of system complexity. Scattering is a less complex process than scanning and there are fewer variables to consider in treatment planning. The beam is shaped to the target volume using apertures and compensators. With scanning, clinicians have more flexibility to shape the beam; but with this sophisticated capability comes increased complexity in planning, computation and equipment.