Purpose: To report the trend analysis of an in-house log file-based patient specific quality assurance (QA) method in pencil beam scanning pediatric proton therapy over 5 years from 2016 to 2020.
Methods: In-house log file-based software was implemented as a part of clinical patient specific QA process to complement 2D array absolute dose measurements. The software extracts beam specific information from treatment delivery log and automatically compares spot MU, lateral position of each spot against the intended values from treatment plan to determine any discrepancies in beam delivery. This software monitors spot width for various energies and gantry angles recorded by spot position monitor against the established baseline at the time of acceptance. Since 2016, the software has analyzed 286 pediatric patients, 476 plans, 1182 fields, and more than 22M proton spots.
Results: The trend analysis showed that proton system has been very stable in delivering patient QA fields within the proton energies of 69.4-221.3 MeV and MU range between 0.003 and 1.473 MU per spot. The planned mean and standard deviation (STD) of energy and spot MU were found to be 114.5 ± 26.4 MeV, and 0.0108 ± 0.0096 MU, respectively. The mean and STD of spot MU and position differences between delivered and planned spots were 1.49x10-7 ± 2.35x10-4 MU, and 0.0289/-0.0073 ± 0.0497/0.0447 mm in X/Y axes for random, 0.0126/0.1343 ± 0.1874/0.1758 mm in X/Y axes for systematic positional differences. The mean and STD of spot width comparison were determined as 0.0927/0.0981 ± 0.1368/0.1734 mm in X/Y axes between the acceptance and delivered values.
Conclusion: We developed an in-house tool to extract crucial information about the stability of the proton pencil beam scanning delivery and monitor the system performance since 2016. Each patient’s plan has been verified pretreatment to ensure the delivery accuracy per machine delivery tolerance.