Increase in second primary lung cancers in a nation-wide cohort study from Sweden

Background We describe the occurrence of second primary lung cancers (SPLCs) and their determinant in Sweden. Methods Nation-wide cancer registry from years 1961 to 2021 identified a total of 853 SPLCs. Results The incidence of SPLCs increased almost linearly from 1980 onwards, equally for women and men and approximately equally after the four main histological types. SPLC included adenocarcinoma 63.9%, squamous cell carcinoma (SCC) 19.4%, small cell carcinoma 9.6% and large cell carcinoma 9.1%. The female cumulative probability (CumP) of SPLC after first adenocarcinoma in 10 years reached 0.019, after SCC 0.015 and after small and large cell carcinoma 0.008. The respective CumP for men was 0.013, 0.012, 0.002 and 0.005. While adenocarcinoma was often followed by second adenocarcinoma, after first non-adenocarcinoma SPLCs presented in diverse histologies. Relative risk of SPLC compared with first lung cancer was overall 3.59, higher for women (4.16) than for men (2.99) and approximately equally high after adenocarcinoma and SCC. In patients diagnosed before age 55 years, the relative risk was 6.68 for all, but after female adenocarcinoma it was 9.95 compared with 6.77 for males. The highest relative risks, up to 20-fold, were found after early onset female adenocarcinoma diagnosed after defined T stages. Conclusions Although SPLCs are still rare, their number is increasing rapidly and the relative risks compared with first lung cancer are substantial, qualifying selected groups of high-risk patients, such as patients with early onset adenocarcinoma for early detection by CT screening when/if such tests become available.