SU‐F‐J‐67: Dosimetric Changes During Radiotherapy in Lung Cancer Patients with Atelectasis

Abstract

Purpose:Large geometric changes which occur during thoracic radiotherapy alter normal anatomy and target position and may induce clinically important dose changes. This study investigates variation of organ‐at‐risk (OAR) dose caused by atelectasis resolution during radiotherapy.Methods:3D IMRT treatment plans were obtained for 14 non‐small‐cell lung cancer patients. Dose of the clinical plan was recalculated on a baseline scan in which lung was collapsed and on a midtreatment scan in which lung re‐aeration had occurred. The changes in OAR doses were compared between the two time points. RTOG‐0617 and inhouse dose‐volume constraints were chosen for investigation and included spinal cord, esophagus, heart, and healthy lung.Results:17 dose metrics were evaluated. The mean (SD) of change in mean lung dose, from baseline to mid‐treatment (average taken across all patients), was 0.2 Gy (2.2 Gy) and ranged from −3.2 Gy to 6.0 Gy. 50% of patients experienced relative changes in mean lung dose of greater than 5% of baseline value. The mean (SD) of changes in heart V40, V45, and V60 were 3.2% (3.4%), 3.0% (2.9%), and 1.4% (2.1%), respectively, and were significant for the study cohort (Wilcoxon signed‐rank test, p=0.0107 for V40, p=0.0052 for V45, and p= 0.0353 for V60. Ranges in changes of Heart V40, V45, and V60 were −1.9% to 8.6%, −1.7% to 7.5%, and −2.1% to 4.5%, respectively. The mean (SD) of changes in Esophagus PRV Dmean and V60 were 0.3 Gy (3.3 Gy) and 0.8% (7.7%), respectively, and ranged from −4.8 Gy to 6.8 Gy for Dmean and −15.2% to 14.6% for V60.Conclusion:Patients with atelectasis present at the start of radiotherapy experience significant increases in heart dose. Substantial increases in mean lung dose also occur in a subset of patients.This work supported by the National Cancer Institute of National Institutes of Health under Award Number R01CA166119. Disclosures: Phillips Medical systems (Hugo, Weiss), National Institutes of Health (Hugo, Weiss, Christensen), and Roger Koch (Christensen) support, UpToDate (Weiss) royalties, and Varian Medical Systems (Hugo, Weiss) license. No potential conflicts of interest.