Dosimetric accuracy of AAA and acuros XB dose calculations within an air cavity for small fields of a 6-MV flattening filter-free beam

The purpose of this research were to verify the accuracy of dose calculation by using the analytical anisotropic algorithm (AAA) and Acuros XB (AXB) against measurements within an air cavity for small fields from a 6-MV flattening filter-free (FFF) beam. A rectangular slab phantom containing an air cavity was specially constructed for this study. Computed tomography (CT) image data sets were acquired with and without a film at four selected depths (4.5, 5.5, 6.5, and 7.5 cm) in the air cavity of the phantom for the dose calculation. The central-axis point dose (CAPD) and dose profile were measured with the film at selected depths for field sizes from 2 x 2 to 5 x 5 cm(2) along the central-beam axis of a 6-MV FFF beam. The central-axis doses (CADs) and dose profiles calculated by using both the AAA and AXB were obtained by using an Eclipse treatment planning system (TPS) under the same measurement conditions. The calculation algorithms were denoted as AXB_w and AAA_w when the film was included in an air cavity of the phantom and an AXB_w/o and AAA_w/o when no film was included. The accuracy of the CAD and dose profile calculated by using both algorithms were compared with the measured CAPD and dose profile, and their differences were evaluated by using root-mean square-error (RMSE) and gamma-index analyse. The percentage difference (%Diff) of the CAD calculated by using AXB w showed good agreement (within 5%) with the measured CAPD at selected depths in the air cavity. However, the corresponding values for the other algorithms, especially AAA_w and AAA_w/o appeared to exhibit relatively high disagreement. The maximum %Diffs between the calculated CAD and the measured CAPD were 4.8% and -39.4% for the AXB_w and AXB_w/o, respectively. The %Diffs increased with decreasing field size and increasing measurement depth. For the calculated and the measured dose profiles, the RMSE values for AXB_w were within 9.3 cGy in both the inner profile and the penumbra whereas the RMSE values for AAA_w produced a wide range (52.2 - 96.8 cGy). This study demonstrated that the dose calculated by using AXB was more accurate than that calculated by using the AAA when compared to the measured dose in the air cavity. In addition, we observed that AXB_w was superior to AXB_w/o in this region with respect to the measurements.