[Impact of Irradiation Field Aperture Controller Settings on Patient-specific Quality Assurance].

PURPOSE: The aperture shape controller (ASC) is a tool on a radiation therapy planning system to reduce complexity by increasing the aperture size of multileaf collimator (MLC). The purposes of this study were to clarify the effect of the dose index on the treatment plan when the intensity of ASC is changed and then to clarify the effect on the verification result in the individual patient-specific quality assurance (QA) using the verification phantom. METHODS: For four types of mock structures presented at AAPM TG-119, volumetric modulated arc therapy (VMAT) treatment plans with three dose levels were set without using the ASC for each of these four types. ASC settings were changed to very low, low, moderate, high and very high for the treatment plan, and the treatment plan was recreated without changing the planning target volume (PTV) and/or OAR dose constraints. The dose index of the treatment plan was then evaluated. The plan was also transferred by the true composite method to the assessment phantom, and patient-specific QA was evaluated. RESULTS: The frequency of dose constraint deviation by changing the ASC was 3.6% (2/55). In patient-specific QA, when the ASC setting was set to moderate, the γ analysis passing rate increased by 1.5% compared to the case without the ASC. In the treatment plan with a γ analysis pass rate of 95% or less without ASC, a 2.7% increase in the γ analysis pass rate was observed by setting the ASC to moderate. CONCLUSION: We found that setting the ASC tended to improve patient-specific QA. Compared to when the ASC was not set, the evaluation result of patient-specific QA was improved without violating the dose constraints of the PTV and/or organ at risk (OAR). In particular, the improvement was notable for the case where the evaluation result of patient-specific QA was 95% or less when the ASC was not used.

[Effect of Relative Electron Density and Physical Density Adjustment in Patient-specific Quality Assurance Using PMMA Phantom].

PURPOSE: The purpose of this study was to improve the accuracy of dose-distribution calculations by understanding how the calculated dose varies with the change in the relative electron density replacing polymethyl methacrylate (PMMA) in patient-specific quality assurance. METHOD: We calculated the relative electron density at which dose attenuation in each dose calculation algorithm coincides with the measured value of the dose attenuation of single-field irradiation. Next, the dose change was calculated by changing the relative electron density or physical electron density for substituting PMMA for each X-ray energy and calculation algorithm. Furthermore, using clinical plans, changes in point-dose verification and dose-distribution verification that occurred when the relative electron density or physical electron density was varied were investigated. RESULTS: The dose attenuation varies depending on the dose-calculation algorithm, and the optimum value of the electron density is different for each. After the electron density optimization, the point dose verification using the 97.1% to 98.3% (3%/3 mm), 90.0% to 94.3% (2%/3 mm) and gained a dominant improvement tendency (P<0.001). CONCLUSIONS: We clarified dose change accompanying relative electron density or physical electron density change. We concluded that the accuracy of dose-distribution calculation for verification improves by replacing PMMA with optimal relative electron density or physical electron density.