Empowering Intensity Modulated Proton Therapy Through Physics and Technology: An Overview

Abstract

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.

Proton Therapy for Locally Advanced Oropharyngeal Cancer: Initial Clinical Experience at the University of Washington

“…However, clinical data evaluating tumor control endpoints are still limited. The combination of proton beam range uncertainty, RBE variation, and sharp dose fall-off does argue for careful evaluation of clinical outcomes, including patterns of failure, to ensure there is not an increase in marginal misses [34,35]. While there are questions regarding the cost effectiveness of using proton therapy for head and neck cancers, this topic is beyond the scope of this article [36,37].…”

“…Intensity-modulated radiation therapy (IMRT) with or without concurrent systemic therapy remains the standard of care for most patients with OPSCC [12]. The development of pencil beam scanning implemented on a gantry allows for improved beam modulation, target conformality, and entrance dose reduction [13]. These characteristics can potentially facilitate greater dose sparing of important organs at risk (OARs).…”

What will the medical physics of proton therapy look like 10 yr from now? A personal view

“…However, this problem has been recognized by both customers and vendors alike, and the situation is rapidly changing. With the introduction of CBCT or in‐room diagnostic CT's, more accurate and efficient patient positioning should soon be available for proton therapy, which will hopefully have a significant impact on treatment times per fraction and also a consequent increase in patient throughput …”

“…With the introduction of CBCT or in-room diagnostic CT's, more accurate and efficient patient positioning should soon be available for proton therapy, which will hopefully have a significant impact on treatment times per fraction and also a consequent increase in patient throughput. 14 Nevertheless, patient setup, if performed in the treatment room is, from the point of view of workflow and patient throughput, not necessarily an optimal approach. For instance, every minute spent setting-up and imaging, evaluating and correcting a patient is time when the most expensive part of the facility (the accelerator and gantry beam line) are simply lying idle.…”

Advanced Treatment Planning

“…In terms of mathematical complexity of treatment delivery, particle dose optimization could be conceived as a much simpler problem. Nowadays, the true development challenges lie especially in dealing with treatment‐related uncertainties, giving a boost to the field of robust optimization . Developments that had initially been suggested for photon treatment planning are being assimilated quickly into particle therapy, especially in the fields of automatic planning, plan adaptation, and multicriteria optimization.…”