Uncertainty Assessment: Relative versus Absolute Point Dose Measurement for Patient Specific Quality Assurance in EBRT

Abstract

Introduction Verification of the accuracy of calculated dose distributions is an essential part of ongoing treatment planning system (TPS) quality assurance (QA). All measurement techniques used to verify the TPS calculated dose have an associated uncertainty. A best practice approach to keep the total uncertainty under acceptable limits is to reduce the uncertainty associated with each step of the process. Point dose patient-specific QA is recommended by the American Association of Medical Physicists (AAPM) and the European Society for Radiotherapy and Oncology (ESTRO) for all complex radiation therapy treatment techniques. Method Relative and absolute point dose measurement techniques have a number of steps to measure the point dose; uncertainty associated with each step is estimated. In this study, a rectangular (uniform) distribution was assumed to estimate the type B uncertainties. 13 head and neck (H&N) intensity modulated radiation therapy (IMRT) treatment plans were selected to observe the effect of the uncertainties on the results of relative and absolute point dose measurement techniques. Results The uncertainties associated with relative and absolute point dose measurement methods were 1.0 and 2.1% (k = 1) respectively. The average percentage point dose differences between the TPS computed dose and measured (absolute and relative) doses were 1.4% and 1.0% respectively, while the average percentage difference between relative and absolute point dose measurements was 0.4%. Independent sample t-testing of the means of percentage point dose difference for absolute and relative methods showed no statistically significant difference in the absolute (M = 1.37, SD = 0.90) and relative (M = 0.95, SD = 1.20) point dose measurement methods (t (24) = - 0.91, p = 0.32). Conclusion Absolute and relative point dose measurement methods do not show statistically significant differences when assessing dose distributions for head and neck IMRT treatment plans.