SU‐D‐304‐04: Pre‐Clinical Feasibility Study for Intensity Modulated Grid Proton Therapy (IMgPT) Using a Newly Developed Delivery System

Purpose: The purpose of the current study was to characterize and evaluate intensity‐modulated proton grid therapy (IMgPT) using a clinical proton beam. Methods: A TOPAS MC model of a new developmental mode (pre‐clinical) of the Hitachi proton therapy system (PROBEAT) was used for simulation and characterization of proton grid therapy. TOPAS simulations of different energy ranges, depths and spot separation distances were performed. LET spectra for various energies and depths were produced with FLUKA MC code for evaluation potential interplay between planning parameters and their effect on the characterization of areas (valley) between spots. IMgPT planning aspects (spot spacing, skin dose, peak‐to‐valley ratios, beam selection, etc.) were evaluated for different phantom and patient cases. Raysearch software (v4.51) was used to perform the evaluation. Results: Calculated beam peak‐to‐valley ratios scenarios showed strong energy and depth dependence with ratios to be larger for higher energies and shallower depths. Peak‐to‐valley ratios for R90 range and for spot spacing of 1cm varied from 30% (E = 221.3 MeV, depth 30.6 cm) to 80% (E = 70.3 MeV, depth 4 cm). LET spectra calculations showed spectral hardening with depth, which might potential increase, spot separation distance and improve peak‐to‐valley ratios. IMgPT optimization, using constant spot spacing, showed skin dose reduction between peak regions of dose due to the irradiation of less skin. Single beam for bulky shallower tumors might be a potential candidate for proton grid therapy. Conclusions: Proton grid therapy using a clinical beam is a promising technique that reduces skin dose between peak regions of dose and may be suitable for the treatment of shallow tumors. IMgPT may be considered for use when bystander effects in off peak regions would be appropriate.