Proton and Heavy Particle Intracranial Radiosurgery

Abstract

Stereotactic radiosurgery (SRS) involves the delivery of a highly conformal ablative dose of radiation to both benign and malignant targets. This has traditionally been accomplished in a single fraction; however, fractionated approaches involving five or fewer treatments have been delivered for larger lesions, as well as lesions in close proximity to radiosensitive structures. The clinical utilization of SRS has overwhelmingly involved photon-based sources via dedicated radiosurgery platforms (e.g., Gamma Knife^® and Cyberknife^®) or specialized linear accelerators. While photon-based methods have been shown to be highly effective, advancements are sought for improved dose precision, treatment duration, and radiobiologic effect, among others, particularly in the setting of repeat irradiation. Particle-based techniques (e.g., protons and carbon ions) may improve many of these shortcomings. Specifically, the presence of a Bragg Peak with particle therapy at target depth allows for marked minimization of distal dose delivery, thus mitigating the risk of toxicity to organs at risk. Carbon ions also exhibit a higher linear energy transfer than photons and protons, allowing for greater relative biological effectiveness. While the data are limited, utilization of proton radiosurgery in the setting of brain metastases has been shown to demonstrate 1-year local control rates >90%, which are comparable to that of photon-based radiosurgery. Prospective studies are needed to further validate the safety and efficacy of this treatment modality. We aim to provide a comprehensive overview of clinical evidence in the use of particle therapy-based radiosurgery.

Keywords: ablative; arteriovenous malformation; carbon; particle; proton; radiation oncology; radiation therapy; radiosurgery; stereotactic; tumor.

Figure 1

A graphical representation of tissue depth as a function of relative dose with the former on the x-axis and the latter on the y-axis. The purple graph depicts a photon beam, the red—an electron beam, the green—a carbon ion beam, and the blue—a proton beam. Both the proton and carbon ion beams exhibit significant Bragg Peaks, which are absent for photons and electrons. Additionally, the fragmentation tail associated with carbon ions is depicted. Note the absence of a sharp Bragg Peak for photons and its significant tissue penetration. Electron beams are largely used for superficial treatments (e.g., basal cell carcinomas of the skin) and exhibit a rapid dose fall-off and low tissue penetration.

Figure 2

Global distribution of operation proton centers (A) and carbon ion centers (B) as of July 2020. Presently, there are 90 and 12 operational proton and carbon ion centers, respectively. Adopted from the Particle Therapy Co-Operative Group (https://www.ptcog.ch/index.php/facilities-in-operation). Figures generated using mapchart.net.