Bortfeld, director of physics at the radiation oncology department at Massachusetts General Hospital (Boston, MA), explained that the current enthusiasm surrounding the field of proton therapy may possibly be due to photon-based radiotherapy having reached its ceiling of development - the point of diminishing return so to speak. As a result, the time is ripe for proton therapy to step into the fore.
"We are at an exciting stage of development in proton therapy- it is still far away from the ceiling," Bortfeld told World Congress delegates. "But it's also a very critical point in time, because if we make mistakes now, interest may drop to zero." As such, he cited several technology challenges that require a major physics input.
Seizing the benefits
First up, there's a real need to optimize the key physical advantage that particle therapies offer - namely, that the finite range of protons and heavy ions enables highly conformal dose targeting. But to fully exploit this property, said Bortfeld, one must consider that "in a patient, what you see is not always what you get".
For example, if a tumour shrinks during fractionated treatment, this may result in a subsequent overshoot of the beam as it traverses more of the (lower density) healthy tissue. Such uncertainties and risks must be addressed - for example by adding distal margins and using tangential fields to avoid overshoot into critical structures.
It's also important to make the most of the unique opportunity that proton therapy offers to image dose distribution following treatment. This range verification, achieved via PET imaging of positron-emitting isotopes generated in the patient, relies on the PET scan being performed as soon as possible in order to minimize isotope washout. Massachusetts General Hospital, for example, has just implemented in-room PET, which enables scanning to start just two minutes after treatment has finished.
Another challenge is to assess the biological and clinical efficacy of the benefits promised by proton therapy. For example, how does avoidance of the dose bath seen in photon-based treatments actually translate into a clinical benefit? Bortfeld cited an example study in rats showing that addition of even a low dose bath to the parotid gland during treatment led to enhanced function loss, and suggested that a similar effect would likely apply within humans.
Bortfeld also touched upon the somewhat controversial issue of whether clinical trials should be performed in proton therapy. While the details of this dilemma were beyond the scope of his presentation, he noted that should such trials take place, the physicists' role in their design is critical.
The ultimate task for physicists and engineers lies in redesigning proton therapy technology to make the treatment more affordable - and thus more widely available. The introduction of pencil-beam scanning is one prime example, with scanned beams removing the need for expensive brass collimators. The real breakthrough, however, could lie in the development of alternative accelerator techniques.
One example is Still River Systems' development of a compact cyclotron that could ultimately be mounted on a gantry to rotate around the patient. "I expect we'll see more compact single-room proton therapy systems in maybe three years from now," Bortfeld noted. Further in the future, developments such as the dielectric wall accelerator and laser-accelerated particle beams could prove a success.
"There are a number of interesting physics challenges and opportunities in particle therapy at the moment," Bortfeld concluded. Watch this space.
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