Evaluation of the difference between dose-to-water and dose-to-medium for stereotactic ablative radiotherapy (SAbR) with the Acuros XB algorithm focusing on lung and bone using clinical plans

A particular algorithm of recent interest is called AXB. AXB works by solving the Linear Boltzmann Transport Equation (LBTE) using numeric methods to calculate photon doses. It can report the dose as both Dm and Dw. There have been a few studies which have compared AXB using both reporting modes to other algorithms such as Monte Carlo (MC) and anisotropic analytical algorithm (AAA). These have shown that AXB is comparable to MC and superior to AAA especially when it comes to calculating doses at high density gradients.    Fewer studies have looked at the differences between Dw and Dm for AXB for SAbR using clinical plans. The majority of these studies have looked at Dm when using SAbR for lung while those looking at SAbR and bone are rare. Bone is of particular interest as this is where the greatest difference is expected and reducing dose to bone has the implication of reducing the likelihood of bone fractures as a side effect of radiotherapy treatment. For these reasons it is important to further investigate if the difference between Dw and Dm using AXB is significant for bone and lung when using SAbR.    This study will compare Dm calculated by AXB (AXB_Dm) and Dw calculated by AXB (AXB_Dm) for clinical SAbR plans previously calculated using AAA. It will focus on lung and bone cancers to assess the significance of the difference between the two calculation methods.