The risk and distribution of second primary cancers according to subsite of primary stomach cancer: a retrospective cohort population-based study.

Background: The development of second primary cancers (SPCs) following a diagnosis of stomach cancer presents a significant clinical challenge, with varying risks depending on the anatomic subsite of the primary tumor, patient demographics, and treatment modalities. This study aims to assess the risk of SPCs in stomach cancer survivors, focusing on differences across anatomic subsites, sex, age, and treatment periods. Methods: The authors conducted a retrospective cohort study using data from stomach cancer patients, analyzing the incidence of SPCs based on the anatomic location of the primary tumor, with stratifications by sex, age, latency period, and year of diagnosis. Standardized incidence ratios (SIRs) were calculated to compare the observed SPC rates with those expected in the general population. Results: Elevated stomach SPC risk was observed across most anatomic subsites, particularly in the body (SIR 8.84) and fundus (SIR 7.34). Females exhibited higher SIRs compared to males, especially in the fundus (SIR 13.33 for females vs. 4.55 for males). Younger patients (<50 years) had significantly higher SPC risks, particularly for cancers originating in the fundus (SIR 49.56). Notably, patients diagnosed after 2010 showed the highest SIRs, indicating a potential impact of advances in diagnostic and therapeutic modalities. Nonstomach SPCs, including colorectal, lung, and thyroid cancers, were significantly elevated, with distinct patterns based on the primary tumor site. Conclusions: The study highlights the critical role of primary tumor location, sex, age, and treatment era in determining SPC risk in stomach cancer survivors. These findings underscore the need for tailored surveillance strategies to manage long-term cancer risks in this population.