On the sensitivity of common gamma-index evaluation methods to MLC misalignments in Rapidarc quality assurance

In this study the effects of small systematic MLC misalignments and gravitational errors on the quality of Rapidarc treatment plan delivery are investigated with respect to verification measurements with two detector arrays and the evaluation of clinical significance of the error-induced deviations.

Five prostate and six head and neck plans were modified by means of three error types: (1) both MLC banks are opened, respectively, in opposing directions, resulting in larger fields; (2) both MLC banks are closed, resulting in smaller fields; and (3) both MLC banks are shifted for lateral gantry angles, respectively, in the same direction to simulate the effects of gravity on the leaves. Measurements were evaluated with respect to a gamma-index of 3%/3 mm and 2%/2 mm. Dose in the modified plans was recalculated and the resulting dose volume histograms for target and critical structures were compared to those of the unaltered plans.

The smallest introduced leaf position deviations which fail the >90% criterion for a gamma-index of 2%/2 mm are: (1) 1 mm; (2) 0.5 mm for prostate and 1.0 mm for head and neck cases; and (3) 3 mm corresponding to the error types, respectively. These errors would lead to significant changes in mean PTV dose and would not be detected with the more commonly used 3%/3 mm gamma-index criterion.

A stricter gamma-index (2%/2 mm) is necessary in order to detect positional errors of the MLC. Nevertheless, the quality assurance procedure of Rapidarc treatment plans must include a thorough examination of where dose discrepancies occur, and professional judgment is needed when interpreting the gamma-index analysis, since even a >90% passing rate using the 2%/2 mm gamma-index criterion does not guarantee the absence of clinically significance dose deviation.