Background: We aimed to compare the impact of systematic uncertainties in patient setup and prostate motion on three different external-beam radiotherapy protocols for prostate cancer.
Methods: To simulate possible near-maximum systematic errors, the isocenter position was shifted to eight points with +/-1.65 SD of the integrated uncertainty value along each axis that was expected to include 5%-95% of the total systematic uncertainties in each direction. Five cases were analyzed for the three plans: an old three-dimensional conformal radiotherapy (3D-CRT) protocol (four-field plus dynamic arc), a new 3D-CRT protocol (dynamic arc), and an intensity-modulated radiotherapy (IMRT) protocol, respectively.
Results: The averaged percentage volume covered by more than 95% of the prescription dose (V95) of the clinical target volume (CTV) for the original plans was 100% for all protocols. After simulating the errors, V95 of the CTV for IMRT cases was maintained at 100%. On the other hand, these values for the new and old 3D-CRT protocols were 93.1% and 63.2%, respectively. The values for the percentage prescription dose received by at least 95% volume (D95) of the CTV for the original plans were 100%, 98.4%, and 97.6% for the IMRT, new 3D-CRT, and old 3D-CRT plans, respectively. However, when the effects of the systematic errors were taken into consideration, the net decreases in the D95 values were 0.3%, 4.3%, and 8.1%, respectively.
Conclusion: The current IMRT protocol is considered to successfully compensate for systematic uncertainties. In contrast, the multi-leaf collimator (MLC) margins set for the old 3D-CRT protocol were not enough to ensure the actual delivery of the prescription dose to the CTV. Therefore, it is very important to include these issues in the plan design in the interpretation of clinical outcomes.