SU-E-T-384: Evaluation of Dose to Medium Vs. Dose to Water Calcualtion Mode in Acuros XB Algorithm for Lung SBRT

Purpose: The purpose of this study it to evaluate the dose to medium vs. dose to water reporting mode of AcurosXB algorithm in Eclipse TPS for lung SBRT Methods: Five lung SBRT cases previously calculated with AAA algorithm have been chosen for this study. All high density materials within the body were contoured and assigned HU values. The plans were then recalculated with AcurosXB algorithm using both dose to medium (AXM) and dose to water (AXW) modes. DVHs were then calculated to quantify the differences in dosimetric indices. Also, plan differences (AXW-AXM) and (AXW-AXM) were computed to observe the regions where maximum dose differences occur within the body. Results: For the same plan normalization, the MU variation between the plans was within ±0.5%. All dosimetric indices of target were within ±0.5% except maximum dose to 2cm (D2-Max) from the target. D2-Max was consistently higher with AXW plan with a value as high as 2.71%. All the dosimetric indices for both ipsilateral and contralateral lung appear to be within ±1% on average without any clear trend. Likewise, dosimetric indices for heart and Esophagus appear well within ±1% on average. The maximum dose to spinal cord was recorded consistently higher with dose to water reporting mode with an average of 2.2% (range 1.40% to 4.63%). Also, the plan difference (AXW-AXM) consistently produced higher maximum dose difference 8.94% (range, 4.2% to 12.4%) as opposed to 2.82%(range, 2.8% to 3.8%) from plan difference (AXM-AXW). Conclusion: It appears that dose reporting mode in AcurosXB algorithm has insignificant effects in dosimetric indices for SBRT lung. The only significant difference observed was the maximum dose difference within the body which may not be clinically important. This may be due to the fact that AcurosXB algorithm transport the particles in medium regardless of dose reporting mode chosen.