DOSIMETRIC IMPACT OF ACUROS XB DOSE-TO-WATER AND DOSE-TO-MEDIUM REPORTING MODES ON LUNG STEREOTACTIC BODY RADIATION THERAPY AND ITS DEPENDENCY ON STRUCTURE COMPOSITION

Abstract Purpose To assess the dosimetric impact of switching from the analytical anisotropic algorithm (AAA) to Acuros XB (AXB), with dose-to-medium (Dm) and dose-to-water (Dw) reporting modes, in lung SBRT patients; and determine whether planning-target-volume (PTV) dose prescriptions and organ-at-risk (OAR) constraints should be modified under these circumstances. Methods and materials We included 54 lung SBRT patients. We delineated the PTV, the ipsilateral lung, the contralateral lung, the heart, the spinal cord, the esophagus, the trachea and proximal bronchi, the ribs, and the great vessels. We performed dose calculations with AAA and AXB, then compared clinically relevant dose-volume parameters. Paired t-tests were used to analyze differences of means. We propose a method, based on the composition of the involved structures, for predicting differences between AXB Dw and Dm calculations. Results The largest difference between the algorithms was 4%. Mean dose differences between AXB Dm and AXB Dw depended on the average composition of the volumes. Compared with AXB, AAA underestimated all PTV dose-volume parameters (-0.7 Gy to -0.1 Gy) except for GI, which was significantly higher (4%). It also underestimated V5 of the contralateral lung (-0.3%). Significant differences in near-maximum doses (D2) to the ribs were observed between AXB Dm and AAA (1.7%), and between AXB Dw and AAA (-1.6%). AAA-calculated D2 was slightly higher in the remaining OARs. Conclusions Differences between AXB and AAA are below the threshold of clinical detectability (5%) for most patients. For a small subgroup, the difference in maximum doses to the ribs between AXB Dw and AXB Dm may be clinically significant. The differences in dose volume-parameters between AXB Dw and AXB Dm can be predicted with reference to structure composition. Summary It is necessary to assess the impact in prescription and dose-volume reporting to the PTV and OARs when switching from AAA to Acuros XB (Dm and Dw) in lung SBRT. We analyzed systematic differences and inter-patient variability of dose-volume parameters between AAA and AXB for 54 patients. We validated a method to predict differences in dose parameters between Dw and Dm with reference to structure composition. Our results may facilitate decision-making to implement AXB.