Ablative radiotherapy for ultracentral lung cancers: Dosimetric, geometric, and volumetric predictors of outcomes and toxicity

Background: Ultracentral lung cancers arise near the proximal bronchial tree (PBT), trachea, or esopha-gus, and have been associated with worse outcomes and increased toxicity after radiotherapy. We sought to associate dosimetric and anatomic factors with oncologic outcomes and toxicities. Methods: One-hundred ten patients treated with ablative, curative-intent radiotherapy for ultracentral, node-negative, non-small cell lung cancer were included. Dosimetric and geometric data obtained using custom software that calculated volumes of target structures and organs-at-risk and measured the short -est vector length between these volumes were associated with outcomes and toxicity. Results: Common dose/fractionation schemes included 50 Gy in 5 fractions (57%), 60 Gy in 8 fractions (15%), and 48 Gy in 4 fractions (13%). Overall survival at 1, 2, and 5 years was 78%, 57%, and 32%, respectively. Factors significantly associated with death included endobronchial tumor, gross tumor volume (GTV) or planning target volume (PTV) contacting PBT, shorter distance from GTV to PBT or esophagus, volume of PBT receiving prescription dose, higher pericardium max dose, lung V20Gy, and mean lung dose. Local progression at 1, 2, and 5 years was 4%, 16%, and 21%. Factors associated with local progres-sion were lower GTV minimum dose and higher GTV/PTV volume ratio. Acute and late grade 2 + toxicity was seen in 18% and 27%, respectively. Four patients (4%) had fatal toxicity. Conclusions: Lower GTV minimum dose and smaller volumetric PTV expansions may increase risk of local progression, and should be balanced against normal tissue doses including pericardium maximum dose, lung V20Gy, and mean lung dose. (c) 2021 Elsevier B.V. All rights reserved. Radiotherapy and Oncology 158 (2021) 246-252