Dose calculation and reporting with a linear Boltzman transport equation solver in vertebral SABR

Vertebral Stereotactic ablative body radiotherapy (SABR) involves substantial tumour density heterogeneities. We evaluated the impact of a linear Boltzmann transport equation (LBTE) solver dose calculation on vertebral SABR dose distributions. A sequential cohort of 20 patients with vertebral metastases treated with SABR were selected. Treatment plans were initially planned with a convolution style dose calculation algorithm. The plan was copied and recalculated with a LBTE algorithm reporting both dose to water (D_w) or dose to medium (D_m). Target dose as a function of CT number, and spinal cord dose was compared between algorithms. Compared with a convolution algorithm, there was minimal change in PTV D90% with LBTE. LBTE reporting D_m resulted in reduced GTV D50% by (mean, 95% CI) 2.2% (1.9-2.6%) and reduced Spinal Cord PRV near-maximum dose by 3.0% (2.0-4.1%). LBTE reporting D_w resulted in increased GTV D50% by 2.4% (1.8-3.0%). GTV D50% decreased or increased with increasing CT number with D_m or D_w respectively. LBTE, reporting either D_m or D_w resulted in decreased central spinal cord dose by 8.7% (7.1-10.2%) and 7.2% (5.7-8.8%) respectively. Reported vertebral SABR tumour dose when calculating with an LBTE algorithm depends on tumour density. Spinal cord near-maximum dose was lower when using LBTE algorithm reporting D_m, which may result in higher spinal cord doses being delivered than with a convolution style algorithm. Spinal cord central dose was significantly lower with LBTE, potentially reflecting LBTE transport approximations.