Validation of the 6X Acuros algorithm version 13.7 in the presence of inhomogeneities for VMAT treatment planning

Abstract

Purpose the aim of this study was to validate the Acuros®XB (AXB) dose calculation algorithm for a Varian TrueBeam Linac against absolute dose measurements in heterogeneous phantoms. Subsequently, the results were benchmarked against the clinically used Anisotropic Analytic Algorithm (AAA). Methods and materials the algorithm was validated in a homogenous water phantom using MapCheck diode array measurements for a range of field sizes and SSDs. To examine AXB in heterogeneous media, two slab phantoms, representing head/neck and abdominal regions, have been created and absolute dose measurements were performed with GafChromic®EBT2 films. The phantoms simulate water, muscle, bone and free air. The setup was modelled in Eclipse and the dose calculated with AXB and AAA. Results The homogenous phantom results met the gamma criteria of 3%, 2 mm. The planar dose differences between AXB and films after an air gap, for a 6 MV beam, are less than 1% in 3 × 3 cm2 field and 2% in 10 × 10 cm2 field. In the case of a 6MV beam, 5 × 5 cm2 field, interacting with a phantom of different densities and an air gap, AXB underestimates the dose after the free air gap, while AAA overestimates the dose. Measurements at 5 mm from the air/water interface show a difference in dose of 3% for AXB and 7% for AAA. Conclusion AXB was validated in a homogeneous phantom. AXB has advanced modelling of lateral electron transport that enables a more accurate dose calculation in heterogeneous regions. As a result, AXB can replace AAA for dose calculations for head/neck and abdominal VMAT plans.