Room: Exhibit Hall | Forum 7
Purpose: GRID therapy has been shown to reduce skin and OAR toxicities. In this study, we use a commercial TPS with a clinical pencil beam scanning (PBS) beam to generate proton GRID plans by optimizing the pencil beam spot placement and beam configuration. Our goal was to reduce equivalent uniform dose (EUD) proximal to the target while maintaining target coverage.
Methods: PBS broad beam and GRID plans were generated in RayStation TPS on phantoms. The spot spacing and interlacing configuration were systematically modified to maximize the pencil beam separation while maintaining target coverage. Robustness criteria were included in the optimization. Plan comparisons were made by varying the number of GRID beams (two to four) and by introducing range shifters (5 cm air gap) to minimize spot size. Benchmarking of GRID plans was performed against broad-beam PBS plans with standard spot spacing. Plan quality was evaluated based on target coverage and normal tissue EUD.
Results: GRIDs arranged orthogonal to the superior-inferior axis produced the best plans. There was an inherent tradeoff between target coverage and normal tissue EUD as the spot spacing is modified, where a larger spacing produced a lower EUD but degraded the target coverage and robustness. GRID plan target coverage was improved by the addition of a range shifter, which enabled spot sigma of 3-6 mm. While two GRID beams provided inadequate target coverage, adding one broad beam or two grid beams produced comparable target coverage to two-field broad beam plans. The normal tissue EUD improvements increased with fraction dose.
Conclusion: We showed that clinical proton PBS beams can be used to deliver GRID therapy and reduce EUD proximal to the target. Further investigation is required to optimize the dosimetry of proton GRID therapy.