Comparison of AAA and Acuros XB Dose Calculation Algorithms for SRS

Single isocenter multiple target (SIMT) stereotactic radiosurgery (SRS) provides an effective treatment of brain metastases. However, the delivered dose to the target and to the surrounding tissue affect SRS outcomes. The accuracy of dose calculation is the key challenge in SIMT SRS application. Two dose calculation algorithms, analytical anisotropic algorithm (AAA) and Acuros XB, have been implemented in a commercial treatment planning system (TPS), Varian Eclipse. The purpose of this project is to compare SIMT SRS dose distribution obtained by Acuros XB dose calculation algorithm and commonly used AAA and to investigate the effects of size, distance to isocenter and heterogeneity.

40 clinical cases with 189 targets were used to evaluate the dose distribution differences. All plans were generated using the Eclipse treatment planning system (TPS) v13.6 and calculated using Analytical Anisotropic Algorithm (AAA). Each patient plan consisted of 2 to 14 targets and treated using volumetric modulated arc therapy (VMAT). These plans were recalculated for the purpose of this project using Acuros XB using the same geometry, voxel resolution and monitor units. Parameters used for plan comparison included planning tumor volume (PTV) coverage to 99%, 95%, and 1%, PTV minimum dose, PTV mean dose, PTV maximum dose, and whole brain V3Gy, V6Gy and V12Gy. The dosimetric accuracy was evaluated based on gamma pass rate with threshold criteria 3%/1mm on SRS MapCHECK.

The two algorithm comparison results showed large dose discrepancies in the PTV minimum dose (-7.9% to 5.3%), occurring also in small field size range and in the small range of distance from PTV to skull. The average difference for D1%, D95% and D99% were 0.8%, 0,2% and 0.03%, respectively. However, the difference of D99% revealed up to -6.1%. Relative to Acuros XB, V3Gy, V6Gy and V12Gy decrease by 1.3%, 0.2% and 2.3% with AAA. Gamma analysis demonstrated higher gamma pass rate for AAA compared to Acuros XB (99.9%,97.9%).

Sizable dose difference were found in AAA and Acuros XB, particularly in the PTV minimum dose and PTV coverage to 99%. Heterogeneity and tumor size introduced uncertainty for dose calculation. However, the dose difference showed no dependence on the distance from isocenter to lesions. AAA showed better agreements between calculated and delivered planar dose distributions than Acuros XB.