Our journeys through the fascinating world of proton radiation therapy

Abstract

The purpose of this article is to share the excitement of the science of proton therapy, told by two physicists, who started their career in this area at different times. The authors' journey spans the evolution of proton therapy over the past 30 years, taking the reader from the time when it was an extremely exotic treatment modality until its more common use today. Over this time period, the authors' research and development aimed at an improved understanding of the physical benefits of intensity-modulated proton therapy and arc therapy, treatment planning and optimization to take proton-specific uncertainties into account, and imaging to measure the proton range in the patient. The final section focuses on emerging themes to democratize proton therapy by substantially reducing its size and price, for much greater affordability and global availability of this treatment modality.

Keywords: adaptive therapy; arc therapy; democratization of proton therapy; image guidance; intensity-modulated proton therapy; proton therapy; range verification; robust optimization.

Figure 1:

Theoretical limit of the dose shaping potential with photon and proton beams for a tumor target volume that wraps around a critical structure. Arc therapy and intensity modulation was assumed in both cases. Note that no range uncertainties were considered in the proton dose distribution. Source: Bortfeld, Habilitation thesis 1995.

Figure 2:

Functional form of the Bragg curve (solid line) derived from first principles, as a convolution of a power-law function describing the stopping power at depth z, with a Gaussian range straggling distribution. 𝒟_−1/2(z) is a parabolic cylinder function, a special function. The dotted curve is without range straggling.

Figure 3:

Schematic drawing (top left) and photo (top right) of the Lunder building at the Massachusetts General Hospital in Boston, where a proton therapy machine was retrofitted in levels LL2 and LL3 below the surface, under the ambulance entrance. Modified from Clasie et al. 2021 [4]. The treatment room is shown in the lower part of the figure (Massachusetts General Hospital photography).

Figure 4:

The patient positioner and immobilization device designed for compact gantry-less proton system. (a) A schematic drawing of three soft immobilization units attached to chair top which is mounted on a robotic positioner base (from Buchner T et al. A soft robotic device for patient immobilization in sitting and reclined positions for a compact proton therapy system. 8th IEEE RAS/EMBS International Conference for Biomedical Robotics and Biomechatronics, 2020:981–988). (b) A volunteer study was performed. The vacuum driven immobilization unit is contracted to the volunteer shoulder and abdominal region. The robotic positioner base is rolled at 8.6 degree. The immobilization units fix the volunteer position on the chair.