Room: Exhibit Hall
Purpose: All proton therapy plans for treatment at our institution using the Mevion S250 passive scattering therapy system are verified for correct monitor units by patient-specific dosimetric measurement. We describe software methods that are designed to streamline the workload, improve accuracy, and reduce the possibility of errors in the overall process of plan quality assurance and monitor unit determination.
Methods: The RayStation version 6 treatment planning system (TPS) incorporates a complete implementation of the Python scripting language. The scripting interface allows for directly writing custom spreadsheets that are used in each of the QA measurements. The measurements are performed using the IBA MatrixX detector and â€œmyQAâ€? software. To aid in the accuracy of the measurements, a predictive starting monitor unit is extracted from the TPS. A large set (over 500) of measurements were made, covering all of the 24 beam options, to determine the correlation of the plan â€œmetersetâ€? with the actual, measured, clinically valid monitor unit.
Results: The relationship between the treatment plan meterset and the clinically validated treatment monitor unit has been shown to be linear with R-factorâ€™s in excess of 0.999. The measured linear function is incorporated into the software that generates the patient QA spreadsheet, to give a starting point for the QA measurement that will be close to the final value, improving the accuracy of the measurement. A limitation of the commercial software is that regions-of-interest (ROIâ€™s) used in analysis can only be rectangular in shape. To overcome this limitation, and provide additional analysis information, a software tool was written that can compare the measured and TPS generated dose distributions with an arbitrary, user defined, ROI.
Conclusion: Using custom produced software can improve both the efficiency and accuracy of patient-specific quality assurance of double-scattering (aperture and compensator) treatment plans.