Association of conformality index and post-treatment radiation pneumonitis in early-stage non-small cell lung cancer treated with stereotactic body radiotherapy

Abstract

Conformality index of the 50% prescription isodose volume (CI50) is an important variable in lung stereotactic body radiotherapy (SBRT) planning but has not been previously correlated to radiation pneumonitis (RP). We hypothesized that adherence to recommended CI50 for patients undergoing lung SBRT would result in decreased incidence and/or severity of RP compared to that for patients with minimal deviations and unacceptable deviations. We retrospectively identified patients treated between 2006 and 2016, with > 3 months follow up from lung SBRT treatment. CTCAE v4.0 toxicity grades were used to classify RP. Clinically significant RP was defined as grade ≥ 2 toxicity. Using Radiation Therapy Oncology Group CI50 planning guidelines, patients were separated into three groups: acceptable, minor deviation, and unacceptable deviation. CI25 and CI75 values in patients with and without clinically significant RP were also reported in this study. One hundred seventy-four patients with a median follow-up time of 26.8 months were included in this analysis. Overall incidence of grade ≥ 2 RP was 12.7%. Thirty-eight (21.8%) patients had acceptable CI50, 100 (57.5%) had minor deviations, and 36 (20.7%) had unacceptable deviations. Incidence of RP did not significantly differ between patients with acceptable, minor deviation, and unacceptable CI50. Additionally, CI25 and CI75 did not significantly differ between patients with and without clinically significant RP. Adhering to recommended CI50 values does not significantly decrease the incidence of clinically significant RP in patients with NSCLC treated with SBRT. To the authors’ knowledge, this observation has not been published previously.