Precision in Wedge Off-axis Using Independent Dose Verification

It is essential for quality assurance to verify the safety of each individual patient's plan in radiation therapy. The tolerance level for independent verification of monitor unit calculations for non-IMRT clinical radiotherapy has been shown in the AAPM TG114. Thus, we investigated the precision of independent MU (dose) verification considering a wedge off-axis calculation and we conducted a study at twelve institutes for independent verification with the wedge off-axis calculation. The results obtained with the wedge off-axis calculation showed better agreement with the treatment planning system calculation results than those without the former calculation in a phantom study and in the patient retrospective study. The confidence limits with the wedge off-axis calculation were 2.2±3.4% and 2.0±4.3% for the plans with a physical wedge and a non-physical wedge in the patient study, respectively. However, the confidence limits were over 5% without the off-axis calculation. From our multi-institutional study, the results suggested that the tolerance level for the wedge off-axis plan would be 5% when considering the wedge off-axis calculation and the level was similar to that of the treatment planning system using other conventional irradiation techniques.