3209 - Beam-Splitting and Pseudo-Proton-ARC Planning Methods for Spatially Fractionated Proton Therapy on a Compact Proton System

Purpose/Objective(s): In a Phase I clinical trial (NCT05831579), spatially fractionated (GRID) proton therapy was delivered using a collimating brass aperture to achieve a customizable spatial dose distribution, escalating dose to small volumes of bulky tumors. However, use of the brass aperture limits flexibility in high dose placement and the beam arrangement. Two non-brass-aperture proton planning approaches, Beam-Splitting (BS), and Pseudo-Proton-Arc (PPArc), are developed and proposed to improve peak-to-valley-dose-ratio (PVDR) and dose conformality compared to proton GRID therapy using a brass aperture.

Materials/Methods: BS and PPArc lattice plans were created for 9 patients with gross tumor volumes (GTVs) from 259-1231 cc. Dose spheres (PTV_6670) were generated within the GTV via scripting, and planned for 66.7 GyRBE dose in 5 fractions, with 4-12 spheres per patient. The whole PTV was prescribed 20 GyRBE. In BS method, a total of 8 fields were placed, with 4 fields utilizing the integrated adaptive aperture fit directly to PTV_6670, and 4 fields covering the remaining PTV_2000. After final optimization, the PTV_6670 subfields and larger PTV_2000 fields with matching gantry/couch angles are merged. In PPArc method, the same optimization objectives were used, with 2-4 arcs consisting of 12 – 16 beams with 15 degrees spacing.

Results: PTV_2000 V20Gy >= 98% was achieved for all plans. PTV_6670 D95% was found to be 95.5±2.1% and 93.3±1.6% in BS and PPArc methods respectively. Average skin Dmax was 33.9±4.0Gy and 32.4±3.9Gy. GTV PVDR, defined as the ratio of the Dmean of PTV_6670 to that of PTV_Avoid, where PTV_Avoid represents the alternating nearest-neighbor spheres to PTV_6670, was 2.7±0.3 and 2.6±0.2, respectively. Furthermore, the OAR constraints in the Phase I photon lattice clinical trial (NCT04133415) were met in all plans.

Conclusion: Dose coverage objectives were met for all 18 proton lattice plans utilizing the integrated adaptive aperture (AA) only. The semi-automated methods developed in this work achieved high-quality lattice plans with increased flexibility in beam arrangement and plan design. BS has fewer beams and better PVDR comparing to PPArc, but higher MUs needed due to the lateral-dose-fall-off control by using the AA. PPArc has less optimization time and skin dose, but is not yet available for clinical use on this compact proton system.