Proton GRID and LATTICE treatment planning techniques for clinical liver SFRT treatments

This study aims to develop and evaluate various treatment clinical proton spatially-fractionated-radiotherapy (pSFRT) planning techniques namely proton GRID (pGRID) and LATTICE (pLATTICE). Approach: pSFRT plans (27Gy(RBE), single-fraction) were initially developed using phantom geometries and created retrospectively for a liver patient previously treated with photon virtual GRID (vGRID). pGRID plans varied in cylinder diameters (Dcyl=4-14mm) and center-to-center distances (c-t-c=1.7-3.4cm) and were evaluated by peak-to-valley dose ratios (PVDRs), equivalent uniform dose (EUD), and V27Gy. Dcyl and c-t-c distances matching the vGRID EUD and V27Gy values guided pLATTICE sphere distributions. Various pLATTICE techniques, including different beam numbers, orientations, and sphere arrangements, were investigated. We also explored using collimating brass apertures to enhance the pGRID PVDR. Main results: pGRID plans with 3.4cm c-t-c and 4mm Dcyl resulted in 2.09% V27Gy, closely matching vGRID's 1.50%. The resultant pGRID PVDR was 8.92 compared to vGRID's 2.7-3.0. PVDRs were affected by spot sizes with reductions of 15.0% with range shifters and 76.0% from 7.5cm to 27.5cm depths. The highest PVDR of 4.17 was achieved with two-field pLATTICE plans with favorable beam angles, with a 44.6% reduction with unfavorable beam orientations and up to 24.7% reductions with an increasing number of beams. Non-maximal pLATTICE sphere packing arrangements can increase the PVDR with a decrease in V27Gy warranting further investigation. pSFRT plans reduced the healthy liver V5Gy by 83.6%-90.7% compared to vGRID. Apertures can enhance the PVDR by 170% at the deepest depths but will increase the skin D0.03cc from 26.77Gy to 54.66Gy. Significance and conclusion: We developed pGRID and pLATTICE plans, demonstrating that desired plan metrics was achieved by adjusting the geometrical arrangements of SFRT contours. The relative orientation of these contours with beam entrances is crucial for high-quality SFRT plans. pSFRT plans achieved superior PVDRs and better dose sparing to OARs compared to vGRID plans. .