The development of patient-specific quality assurance (PSQA) for a proton wobbling nozzle using PTSim Monte Carlo code

The aim of this study is to develop a Monte Carlo-based platform for patient-specific quality assurance (PSQA) in the proton wobbling nozzle at Chang Gung Memorial Hospital. To efficiently reduce time usage and enhance simulated precision, this platform required an initial fine-tuning of the Guard volume (GV) and lateral displacement (LD) settings. Different GV sizes combined with LD modes were utilized to estimate depth dose profiles in a water phantom. The simulated central axis depth doses were then analyzed using PTSim and compared to measurement using a PTW MP3 water tank and a Markus ionization chamber type 34045. Treatment parameters for a liver case were extracted from the Eclipse DICOM RT Plan file. Simulated doses were compared to measurements using Zebra and MatriXX systems during PSQA process. Furthermore, a geometrical trigger technique was applied to assess the track positions in patient-specific components. The study found that the mean dose difference between measured and simulated values for the optimized LD/GV was below 1% at all depth positions. With passing rates of 97.4% at the middle SOBP and 90.3% at the proximal SOBP, the gamma analysis demonstrated satisfactory results. For the axial dose profile, SOBP flatness was within 2.5%. From the geometric trigger analysis, more than 95–98% of the dose was delivered through the range compensator, while approximately 2–5% originated from scattered protons from the Multi-Leaf Collimator (MLC) and block's edges. The PTSim simulation's performance in conducting PSQA demonstrated its capability and reliability.