Dosimetric validation of Acuros ® XB with Monte Carlo methods for photon dose calculations

Purpose: The dosimetric accuracy of the recently released Acuros ® XB advanced dose calculation algorithm (Varian Medical Systems, Palo Alto, CA) is investigated for single radiation fields incident on homogeneous and heterogeneous geometries, and a comparison is made to the analytical anisotropic algorithm (AAA).Methods: Ion chamber measurements for the 6 and 18 MV beams within a range of field sizes (from 4.0 × 4.0 to 30.0 × 30.0   cm 2 ) are used to validate Acuros ® XB dose calculations within a unit density phantom. The dosimetric accuracy of Acuros ® XB in the presence of lung, low‐density lung, air, and bone is determined using BEAMnrc/DOSXYZnrc calculations as a benchmark. Calculations using the AAA are included for reference to a current superposition/convolution standard.Results: Basic open field tests in a homogeneous phantom reveal an Acuros ® XB agreement with measurement to within ±1.9% in the inner field region for all field sizes and energies. Calculations on a heterogeneous interface phantom were found to agree with Monte Carlo calculations to within ±2.0%( σ MC = 0.8 % )in lung( ρ = 0.24   g   cm − 3)and within ±2.9%( σ MC = 0.8 % )in low‐density lung( ρ = 0.1   g   cm − 3) . In comparison, differences of up to 10.2% and 17.5% in lung and low‐density lung were observed in the equivalent AAA calculations. Acuros ® XB dose calculations performed on a phantom containing an air cavity( ρ = 0.001   g   cm − 3)were found to be within the range of ±1.5% to ±4.5% of the BEAMnrc/DOSXYZnrc calculated benchmark( σ MC = 0.8 % )in the tissue above and below the air cavity. A comparison of Acuros ® XB dose calculations performed on a lung CT dataset with a BEAMnrc/DOSXYZnrc benchmark shows agreement within ±2%/2mm and indicates that the remaining differences are primarily a result of differences in physical material assignments within a CT dataset.Conclusions: By considering the fundamental particle interactions in matter based on theoretical interaction cross sections, the Acuros ® XB algorithm is capable of modeling radiotherapy dose deposition with accuracy only previously achievable with Monte Carlo techniques.