SU-E-T-470: Comparison of Proton Treatment Planning and Monte Carlo Calculation Using TOPAS for Liver Cancer

To compare Monte Carlo (MC) calculated and planned dose distributions (pencil beam algorithm) for patients with liver cancer treated with proton radiation therapy.

Six patients with unresectable Hepatocellular carcinoma were chosen from the institutional protocol list. We applied the newly developed TOPAS (Tool for Particle Simulation) Monte Carlo (MC) tool and an in-house (mcauto) program, which connects the planning system with the MC. Two beams, typically right lateral (RL) and anterior-posterior (AP), were simulated for each patient with a total prescribed dose of 58 Gy. The calculated absolute dose was determined by separately simulating an SOBP dose in a water phantom for normalization to the prescription dose. The difference between MC and planned dose were calculated and Dose Volume Histograms (DVHs) for the critical organs with non-negligible dose (whole liver, heart, small and large bowel and chest wall) were analyzed.

The resulting dose distributions were in quite good agreement. The main discrepancy in all cases was observed in the lateral penumbrae. These discrepancies can mainly result from the range compensator gradient and tissue composition. The Dose Volume Histograms (DVHs) also presented good agreement between doses for the CTV as well as all the OARs. The difference in D95 ranged from 0.7-1.5 Gy that is translated to 1.3-2.5% of the prescribed dose.

TOPAS Monte Carlo tool presented an efficient and accurate method for dose calculation in liver and to validate clinical treatment planning. Discrepancies with doses calculated using the pencil beam algorithm were seen but were generally quite small.