P04.02 Three Definitions of Normal Lung Volumes for Predicting Radiation Pneumonitis in Advanced NSCLC Treated With Radical Chemoradiation Therapy

Abstract

Chemoradiotherapy followed by durvalumab is the standard treatment for the patients with unresectable stage III non-small cell lung cancer (NSCLC), but Grade 2 or greater radiation pneumonitis (RP2) is a major obstacle of the use of following consolidative immunotherapy. Although several dosimetric parameters were found to be predictive of RP2, including mean lung dose (MLD) and the lung volume receiving ≥20 Gy (V20), very few researches have noted different definition of normal lung volume may affect the predictive value of the dosimetric parameters, especially in the curative-intent volumetric intensity modulated arc therapy (VMAT) planning, and limited data regarding the onset of RP2 in different time windows. The purpose of this study was to investigate associations between clinical and three definitions of normal lung volumes and RP2. We retrospectively analyzed 117 stage III NSCLC patients treated with chemotherapy concurrent or sequential VMAT from January 2015 to May 2019 in Guangdong Lung Cancer Institute. Patients with palliative dose (<60 Gy), poor performance status (≥2) or <6 months follow-up were excluded. The incidence of radiation pneumonitis in lung cancer was counted every two weeks. Dosimetric parameters including MLD, V20 and the lung volume receiving ≥5 Gy (V5) were calculated based on three definition of normal lung which excluding GTV (Lung-GTV), CTV (Lung-GTV) or PTV (Lung-GTV) from the total bilateral lung volume. The different RP2 prediction ability was assessed by the areas under the curve (AUC) of receiver operating characteristic (ROC) curves. Clinical and dosimetric factors regarding radiation pneumonitis were evaluated using repeated analysis of variance. The median age of patients was 60 years and median follow-up was 1.4 years. 79(67.5%) patients received concurrent cisplatin/etoposide (45,38.5%) or carboplatin/paclitaxel (28,23.9%) and 38(32.5%) patients received sequential carboplatin/paclitaxel (29,24.8%).Grade 2 or more radiation pneumonitis were developed in 31 (26.5%) patients. Of these, 18(58.1%) occurred within 12 weeks, 10(32.3%), 3(9.6)developed between 12 to 24 weeks and after 24 weeks respectively. There is significant heterogeneity in MLD, V20 among the various definition methods (all p<.05), average MLD, V20 were 16.3Gy,28.6%; 15.5Gy,27.4% and 14.3 Gy,25.6% when excluding GTV, CTV and PTV, respectively. The best RP2 prediction performance is found in MLD from Lung-GTV method (AUC = 0.688), which is significantly better (P = 0.002) than Lung-CTV method (AUC = 0.672) and Lung-CTV method (AUC = 0.654). RP2 most commonly developed within 12 weeks after radiotherapy. MLD from the GTV exclusion method may be more accurate in predicting clinically significant radiation pneumonitis while V5, V20 or any of the clinical factors are shows no statistical significance.