Purpose: The dosimetric advantages of protons, which include a pronounced Bragg peak and low entrance dose, are desirable traits when considering GRID therapies. The objective of this work was to investigate the applicability of using a dynamic collimation system (DCS), an energy layer specific collimator, to improve PBS GRID treatments by allowing variable grid ratios and to demonstrate the viability of this technique between different cancer cell lines.
Methods: GRID dose distributions were simulated using Geant4 with an experimentally benchmarked IBA UN beam line model and physics list. Individual 4 cm spread-out-Bragg peak beamlets were calculated that were collimated with equivalent square fields ranging from 0.25 cm to 1.0 cm. Treatment planning dose distributions were generated in MATLAB by optimizing the spot spacing and collimation width for various cancer cell types. A linear quadratic model was used to assess the cell survival between malignant and nonmalignant tissues when irradiated under full-field and GRID irradiation conditions.
Results: As expected, the effectiveness of GRID treatments were largely impacted by changes in spot spacing or collimation, as these variables directly influence the resulting dose distribution. However, it is not necessarily the case that maximizing the difference between adjacent low- and high-dose regions resulted in a superior dose-volume effect. In fact, no combination of spot spacing and collimation width was universally optimal but rather depended on the cell radiosensitivity and the desired cell kill fraction.
Conclusion: An optimal GRID dose distribution reflects the malignant and non-malignant cell radiosensitivities at the desired dose level. Given the capacity of the DCS to perform energy-layer specific collimation, it appears that this technology may help tailor treatments towards the specific radiobiology of a patient’s tumor in order to maximize the therapeutic benefits from GRID treatments.
Funding Support, Disclosures, and Conflict of Interest: Research reported in this abstract was supported by the National Cancer Institute of the National Institutes of Health under award number R37CA226518. Dr. Daniel Hyer is a co-inventor on a patent that has been licensed to IBA. The other authors have nothing to disclose.