SU-F-T-417: Dosimetric Evaluation of Dose-To-Medium Vs. Dose-To-Water Using Acuros XB for Lung, Spine and Head-And-Neck Plans.

PURPOSE: This study evaluates the dosimetric differences between dose-to-medium (Dm) and conventional dose-to-water (Dw) reporting using Varian's Acuros XB (AXB) dose calculation algorithm with actual patient treatment plans.

METHODS: Twelve patients were selected: four head-and-neck (HN) (2 Gy × 33-35 fractions), four lung SBRT (10-12 Gy × 5 fractions) and four spine SBRT (6-8 Gy × 5 fractions) cases. All treatment plans were optimized using the Varian Eclipse Progressive Resolution Optimizer (version 13.6) to meet objectives of institutional guidelines. After the optimization process, each plan was calculated using both Dw and Dm options within AXB. The calculated treatment plans were normalized such that the prescribed dose covered at least 95% of the PTV. The dose-volume histograms were evaluated for various dosimetric parameters. The percent difference between the two options is calculated as (AXB_Dw - AXB_Dm)/AXB_Dm *100.

RESULTS: The largest difference between AXB_Dw and AXB_Dm was seen in the mandible structure for HN cases, where AXB_Dw was up to 10% higher than AXB_Dm results (5.3%-10.2%). AXB_Dw was also higher for PTV maximum dose in these cases, ranging from 2.5%-5.7%. Among all other dosimetric parameters in HN, Spine and lung SBRT plans; the differences between AXB_Dw and AXB_Dm were within 5%. We also noted that AXB_Dw was consistently higher (1.8-3.8%) for cord in lung SBRT, and was consistently lower for esophagus/bowel in spine SBRT (-5% - -2.9%).

CONCLUSION: The preliminary dosimetric results from our clinical study show that the selection of either Dm or Dw in AXB is not likely to produce significant dosimetric differences in most clinical cases. However, Dw can be different by up to 10% in bony anatomy compared to Dm. Therefore, Dm is the preferred option to achieve the greatest accuracy in both target and critical structure, providing a smooth transition from conventional to state-of-the-art treatment planning.