SU‐E‐T‐896: Effect of PTV Overlap with Chest Wall on Plan Quality of Rapid Arc Lung Stereotactic Body Radiotherapy (SBRT)

Purpose: For peripheral lungSBRT, the PTV often overlaps with chest wall. Clinically, this heterogeneity causes a non‐uniform dose distribution for Rapid Arc (RA) plans, with hot spots in chest wall and under dosage at PTV/lung interfaces. Examining the effect PTV overlap has on plan quality should allow us to predict cases which will be more difficult to plan. More complex planning may be needed for higher risk overlaps.Methods: An anthropomorphic phantom was CT scanned and PTVs were generated in Eclipse treatment planning system to represent varying volumes of overlap of PTV with chest wall: 0%, 10%, 30%, 50%, 70% and 90%. Plans were prescribed 48 Gy in 4 fractions and heterogeneity corrections applied. Plans used five non‐coplanar arcs, all optimized identically. Plan quality was assessed using conformality index (CI100%, CI50%), maximum dose 2cm outside PTV (D2cm), maximum point dose, homogeneity index (HI), and maximum rib dose. Plans with the poorest CI100% and HI were re‐optimized as base dose plans to investigate if more complex planning would result in plan quality similar to smaller overlaps.Results: 0% overlap had the lowest values for all criteria, except D2cm. The HI and maximum dose increased until 50% overlap, and then gradually decreased. The CI100% increased with increasing overlap. No trend was observed for CI50%, D2cm, and maximum rib dose, with values similar for all overlaps. A maximum increase in hot spot in chest wall, and non‐uniform dose distribution was observed between 30%–70% overlap, when PTV densities were most heterogeneous. Re‐optimized base dose plans for 30%–70% reduced CI100% on average by 7.7%, and HI on average by 3.6% Conclusions: PTV overlap with chest wall affects plan conformality and dose uniformity in RA SBRTlung plans. Overlap will reduce plan quality the most when resulting PTV tissue densities are the most heterogeneous.