Higher dose volumes may be better for evaluating radiation pneumonitis in lung proton therapy patients compared to traditional photon-based dose constraints

Abstract Purpose The dosimetric parameters used clinically to reduce the likelihood of radiation pneumonitis (RP) for lung cancer radiotherapy have traditionally been V20Gy≤30-35% and mean lung dose ≤20-23Gy; however, these parameters are derived based on studies from photon therapy. The purpose of this study is to evaluate if such dosimetric predictors for RP are applicable for locally-advanced non-small cell lung cancer (LA-NSCLC) patients treated with proton therapy. Methods/Materials 160 (78 photon, 82 proton) LA-NSCLC patients treated with chemoradiotherapy between 2011-2016 were retrospectively identified in this study. 40 (20 photon, 20 proton) patients exhibited grade ≥2 RP after therapy. Dose volume histograms for the uninvolved lung were extracted for each patient. The percent lung volumes receiving above various dose levels were obtained in addition to V20Gy and Dmean. These dosimetric parameters and patient characteristics were evaluated with univariate and multivariate logistic regression tests. Receiver Operating Characteristic (ROC) curves were generated to obtain the optimal dosimetric constraints through analyzing RP and non-RP sensitivity/specificity values. Results The multivariate analysis showed V40Gy and Dmean to be statistically significant for proton and photon patients, respectively. V35Gy-V50Gy were strongly correlated to V40Gy for proton patients. Based on the ROC curves, V35Gy-V50Gy had the highest AUC compared to other dose levels for proton patients. A potential dosimetric constraint for RP predictor in proton patients is V40Gy≤23%. Conclusions In addition to V20Gy and Dmean, the lung volume receiving higher doses, such as V40Gy, may be used as an additional indicator for RP in LA-NSCLC patients treated with proton therapy.