Room: Exhibit Hall | Forum 7
Purpose: To commission and validate a facility-specific pencil beam scanning (PBS) model using the TOPAS Monte Carlo (MC) code, followed by application of the model for absolute dose verification in patient specific quality assurance (PSQA).
Methods: A facility-specific proton PBS model for MC simulation was developed based on previously measured beam data for commissioning of the treatment planning system (TPS). Simulation started at the nozzle exit. Parameters such as energy layers, spot positions and scanning angles were extracted or calculated based on RTPlans by using the in-house developed script to generate TOPAS input files automatically. This model was initially tuned and verified by comparing in-air spot profiles and in water integrated depth dose (IDD) with measured data. The absolute dose of spread-out-Bragg-peaks (SOBP) and lateral profiles were then calculated and compared with measurements and TPS calculations. Through implementing the commissioned model, two PSQA plans were investigated to compare the field-by-field 2D planar dose distributions with MatriXX PT measurements.
Results: In-air spot lateral profiles from the TOPAS were found to agree with the maximum dose difference within 2%. IDDs were also in good agreement with measured data within 3% of difference. The absolute SOBP doses and lateral profiles were reproduced with the difference less than 5%. The 2D 3%/3mm gamma analysis was performed for both plans. The passing rates for all 3 fields of the brain case were higher than 98%. For the breast case with 2 fields, the passing rates were 95.7% and 99.2%, respectively.
Conclusion: A MC based PBS model has been developed and validated, and the scripting tool has been implemented for generating the model input. This combination scheme will be feasible as an independent check for PSQA results in our facility.