Risk of second primary cancer and death in patients with idiopathic inflammatory myopathies

OBJECTIVES: To assess the risk of second primary cancer and death in patients who has a cancer diagnosis following idiopathic inflammatory myopathies (IIM). METHODS: Using nationwide Swedish healthcare and population registers, we identified 2346 patients diagnosed with IIM ≥18 years of age between 1998 and 2024. Patients were followed for up to 27 years for first and second post-IIM primary cancers and death. Cumulative incidence function and incidence rate of second primary cancer and death were estimated. A flexible parametric multistate model was used to estimate transition probabilities to cancer and death. Furthermore, risk and prognostic factors for second primary cancer and subsequent death were assessed using multivariable models. RESULTS: Of 326 patients diagnosed with cancer at or after IIM diagnosis, 37 developed a second primary cancer during a mean follow-up of four years (maximum 19 years). The cumulative incidence of second primary cancer over the entire follow-up period was 22% (95% CI 14-33%) while the absolute risk within four years after a first post-IIM cancer was below 5%. Being male (hazard ratio, HR=3.2, 95% CI 1.6-6.5) and having a cancer history (HR=3.4, 95% CI 1.5-8.0) were significant risk factors of second primary cancers. The three-year all-cause mortality risk after a second primary cancer was 46% (95% CI 44%-70%). Patients with dermatomyositis had a markedly elevated mortality risk following a second primary cancer diagnosis compared to non-dermatomyositis subtypes (HR=3.77, 95% CI 1.46-9.75). CONCLUSION: These findings provide new insight into the burden of subsequent cancers among IIM cancer survivors and highlight the need for close clinical monitoring for patients with dermatomyositis to reduce premature mortality.