Considering the clinical potential of protons attributable to their physical characteristics, interest in proton therapy has increased greatly in this century as has the number of proton therapy installations. Until recently, passively scattered proton therapy (PSPT) was used almost entirely. Notably, overall clinical results to date have not shown convincing benefit protons over photons. A rapid transition is now occurring with the implementation of the most advanced form of proton therapy, the intensity-modulated proton therapy (IMPT). IMPT is superior to PSPT and IMRT dosimetrically. However, numerous limitations exist in the present IMPT methods. In particular, compared to IMRT, IMPT is highly vulnerable to various uncertainties.
In this overview we identify three major areas of current limitations of IMPT: treatment planning, treatment delivery, and motion management, and discuss current and future efforts for improvement. For treatment planning, we need to reduce uncertainties in proton range and in computed dose distributions, improve robust planning and optimization, enhance adaptive treatment planning and delivery, and consider how to exploit the variability in the relative biological effectiveness (RBE) of protons for clinical benefit.
The quality of proton therapy also depends on the characteristics of the IMPT delivery systems and image-guidance. Efforts are needed to optimize the beamlet spot size for both improved dose conformality and faster delivery. For the latter, faster energy switching time and increased dose-rate are also needed.
Real-time in-room volumetric imaging for guiding IMPT is in its early stages with CBCT and CT-on-rails, and continued improvements are anticipated. In addition, imaging of the proton beams themselves using, for instance, prompt gamma emissions, is being developed to determine the proton range and to reduce range uncertainty.
With the realization of the advances described above, we posit that IMPT, thus empowered, will lead to substantially improved clinical results.