Proton radiography for relativistic proton beam therapy

Proton beam radiation therapy is at the forefront of modern techniques for cancer treatment, due to its high level of radiation-dose deposition accuracy. This treatment accuracy, however, is limited by the ability to position the treatment beam within the patient's anatomy. Typically, the patient's position is registered orthogonally, using X-ray imaging. However, if instead, beam's-eye-view imaging were enabled by proton radiography, dose deposition measurements could be improved with intrinsically-registered patient positioning. At a typical treatment facility, with a maximum proton energy on the order of 250 MeV, imaging capabilities are limited by the high degree of multiple-Coulomb scatter protons accumulate before exiting the patient. However, in increasing the proton energy from 250 MeV to 800 MeV, the accumulated scatter is reduced by a factor of five, and, coupled with a magnetic-lens, collimated imaging system, enables high-resolution, high-contrast imaging. Further, based on results from Geant4, the dose profile of this higher energy beam is tightly constrained, a fact which may be exploited in the future to further increase treatment accuracy. The full-width half-max of the dose-deposition kernels at 33.4 cm depth (the 250-MeV Bragg peak) are 1.56 cm for 250-MeV protons, compared with 0.52 cm for 800-MeV protons. At 1-cm downstream of this point, the 250-MeV kernel has dropped off to 32%, while the 800-MeV dose is still at 95%. An 800-MeV proton treatment plan exploiting the constrained lateral profile would utilize techniques developed for photon therapy, to deliver the dose from 360° and tightly constrain the dose, stereotactically.