SU‐F‐T‐543: the Impact of Density Overrides in IMRT and VMAT Planning for Lung Cancer Patients Treated SBRT

Purpose:To evaluate the dosimetric impact of target volume overrides on the quality of IMRT and VMAT lung, SBRT treatment plans.Methods:Ninety one patients previously treated with lung SBRT at our institution were retrospectively studied. IMRT and VMAT plans were created on 5 image sets for each patient: free breathing scans(FB), average scans(AVE), average scan with internal target volume overridden to tumor density(ITVoverride), average scan with planning target volume overridden(PTVoverride), and a hybrid method where the internal target volume was set to tumor density and the set‐up margin was set to an intermediate density between lung and tumor(HBoverride). All plans were optimized with the same constraints as the original clinical plan AVE. Dose fractionation scheme included 12Gyx4, 10Gyx4, 8Gyx4, 12Gyx3, and 9Gyx5. EQD2 was computed for various target metrics (mean, D1%, D99%) and normal lung metrics (V5Gy, V20Gy, mean lung dose). Plan quality was investigated via conformity index (CI) and gradient index (GI).Results:The ITVoverride plans achieved the highest mean target EQD2(0.38±0.69Gy over AVE) and the highest D1% hotspot(0.73±1.17Gy over AVE) in comparison to all other plans. PTVoverride and HBoverride also demonstrated higher mean target EQD2 than AVE, but lower hotspots. Compared to AVE, PTVoverride and HBoverride resulted in lower coldspots of −9.95±3.78Gy and −9.91±3.68Gy respectively. All override plans achieved lower lung metrics than the AVE plans, with the largest reductions in PTVoverride (−0.31±0.30Gy for V20Gy, −0.44±0.44Gy for V5Gy, −0.23±0.18Gy for mean lung dose). Comparing FB (largest CI,GI) to PTVoverride (smallest CI,GI), CI and GI improved from 1.05 to 1.00, and 4.81 to 4.48 respectively. The differences in these metrics did not correlate with tumor motion or target volume.Conclusion:Despite the improvements in CI and GI, density overrides in target volumes for 91 lung SBRT patients showed limited dosimetric advantages over clinical plans on the average CT.