Evaluation of Dose to Medium and Dose to Water with the Acuros XB Algorithm for SABR: Clinical and Phantom-Based Observations

Objective: Acuros XB (AXB) is a grid-based Boltzmann transport solver that can report dose either to medium (Dm) or to water (Dw). While Dm offers a more physically descriptive model of tissue interactions, Dw maintains continuity with established dosimetry protocols. This study investigated the impact of dose reporting mode on stereotactic ablative body radiotherapy (SABR) planning and verification. Approach: Fifteen previously treated SABR cases (five nodal, five bone, five lung) were retrospectively selected and recalculated using AXB in both Dm and Dw reporting modes. Dose–volume histogram (DVH) metrics for dose maximum and organs at risk were compared. Gamma analysis (2%/2 mm, global normalisation) was performed to assess spatial agreement. Independent verification was undertaken by using anthropomorphic phantoms with point dose ionisation chamber measurements. Main results: In soft tissue and lung, differences between Dw and Dm were negligible (< 2% in DVH metrics, within ±4% of ionisation chamber phantom measurements), and all plans achieved high gamma pass rates (≥ 95% at 2%/2 mm). In bone larger discrepancies were observed, with DVH deviations up to 5.4% and reduced gamma pass rates. Dm differed in bone-equivalent material from phantom measurements by up to 7.7%, whereas Dw showed closer agreement with measurements. Significance: The choice of AXB reporting mode has tissue-dependent implications for SABR. While differences between Dm and Dw are generally small in soft tissue and lung, larger discrepancies may arise in bone and bone-equivalent materials, particularly in high dose gradient regions. Dw demonstrated closer agreement with anthropomorphic phantom verification measurements in bone-equivalent materials than Dm, which showed larger deviations from measured dose than typically accepted SABR verification tolerances. These findings highlight the importance of considering both tissue composition and verification methodology when interpreting dose calculations using AXB.