To Assess the Impact of Bowel Gas Override On Pelvis Treatment Plans: Case Studies
Presentations
PO-GePV-T-238 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: To assess the impact of bowel gas override on pelvis treatment plans as a contributor to overall treatment delivery uncertainty.
Methods: Two VMAT pelvis plans for cervical carcinoma, on cases with noticeable bowel gas (BG) volumes (P1, 11.3% and P2, 27.3% gas in bowel bag) were optimized with mass density override (DO) of 0, 0.5 and 1 g/cc to get clinically acceptable plans. Each optimization was re-calculated with remaining 2 BG-DO overrides to assess the robustness of the optimization on varying bowel-gas conditions in-vivo. PTV coverage metrics of V100%, Dmax(0.03cc) and Dmin(0.03cc) and OAR metrics of Bowel Dmax(0.03cc) and V40Gy and Sigmoid V45Gy were chosen for comparing the studies.
Results: Impact of BG-DO on PTV (V100) was low - in the worst case scenario of no BGO at optimization and recalculated with DO of 1 g/cc , the coverage dropped 2.2% on P1 and 3.8% on P2. Plans optimized with BG-DO of 1 g/cc are least variant (~0.5%) to changes in bowel gas. Changes in bowel gas with DO of 0.5 g/cc resulted in [-4.6%- 1.3% ] difference in MinD0.03cc within the PTV. Impact of DO had a maximum change of 2% on D0.03cc for bowel. On P2, all 3 optimizations resulted in 4-6% variation on V40Gy for bowel as compared to <1% on P1. Impact of BGO on sigmoid V45 was substantial on both plans resulting in >7% variation. However, Sigmoid V44Gy was less sensitive to changes in bowel gas.
Conclusion: Impact of bowel gas in overall treatment delivery uncertainty is of the order of 2-4% on PTV coverage. It is <2% for bowel itself, and OARs overlapping with PTV and in-plane with bowel should be evaluated on less sensitive metrics for robust planning.
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