Verification of intensity-modulated proton therapy (IMPT) plans for rectal cancer using MCsquare and TOPAS Monte...

This study aimed to compare different intensity-modulated proton therapy (IMPT) plans for rectal cancer obtained from the research-based treatment planning system matRad with dose distributions recalculated using MCsquare and TOPAS Monte Carlo simulations, to verify the use of matRad's Monte Carlo interface for secondary cancer risk assessment following rectal cancer IMPT.

An adult male computational phantom was used to represent a rectal cancer patient. Four IMPT plans were generated using matRad, comprising 2-field (lateral and oblique), 3-field, and 5-field setups for whole pelvic irradiation with 50 Gy(RBE) in 25 fractions. The IMPT plans were recalculated using MCsquare and TOPAS. Dosimetric parameters and gamma evaluation were used for comparison across the calculation techniques.

All global and most local gamma passing rates with 3%/2mm criteria and a 10% low dose threshold exceeded the action limit of 90%, and most comparisons exceeded the tolerance limit of 95%. MatRad tended to underestimate normal organ doses compared to both MC codes. TOPAS gave larger mean organ dose differences from matRad than MCsquare. The maximum difference of mean organ dose was found between matRad and TOPAS for the 2-field oblique plan.

The results of this work supported the implementation of the interface between matRad and MC simulations for in-field and out-of-field dosimetry in research and educational settings.