Does irradiation for initial primary lung cancer affect the risk of metachronous second primary lung cancer?

Several studies have reported inconsistent results about second primary lung cancer (SPLC) after irradiation for initial primary lung cancer (IPLC). The present study aims to assess the effect of ionising radiation on the risk of SPLC. The study population came from SEER database, and included a population-based cohort of 21,397 individuals diagnosed with IPLC between 2004 and 2009 who survived more than 7 years after the initial diagnosis. The first aim was to estimate the risk of SPLC in different periods and the cumulative risk of SPLC. Subsequently, a generalized additive model with Poisson regression analysis and a proportional sub-distribution hazard model was used to determine whether radiation affected the risk of SPLC. Until Dec 2016, there were 488 individuals who developed SPLC, 5368 individuals who died, and there were 15,541 alive individuals, respectively. The risk of SPLC was found to gradually decline with the extent of follow-up time. Age and histology were the two main risk factors of developing SPLC in Poisson regression and competing risk analyses. In Poisson regression analysis, radiation had no significant effect on the risk of developing SPLC (adjusted OR = 0.80, 95% CI 0.54, 1.19, P = 0.28). When considered competing risk of all-cause death, the risk of SPLC in the radiation group was similar to that in the non-radiation group (adjusted sHR = 0.80, 95% CI 0.56, 1.13, P = 0.21). The risk of SPLC was different during different follow-up time. Irradiation for IPLC seemingly did not affect the risk of developing SPLC.