Cancer risk in patients with systemic lupus erythematosus: a population-based cohort study in the Republic of Korea 2004–2021

Systemic lupus erythematosus (SLE) is associated with increased cancer risk. However, the patterns of cancer incidence remain unclear. This study aimed to evaluate the cancer risk in patients with SLE. This population-based cohort study identified 24 241 patients with newly diagnosed SLE between 2004 and 2020 using Korean National Health Insurance Service data. Patients were followed up until cancer diagnosis, death, or December 2021. Standardized incidence ratios (SIRs) were calculated to compare cancer risk between patients with SLE and the general population. Subgroup analyses were performed based on the age at diagnosis, follow-up duration, and use of immunosuppressive agents. Patients with SLE had higher risks of overall [SIR 3.3, 95% confidence interval (CI) 3.2–3.4], solid (SIR 3.1, 95% CI 3.0–3.2), and haematological (SIR 9.8, 95% CI 8.9–10.9) cancers compared with the general population. Among solid cancers, liver cancer had the highest risk, followed by ovarian cancer. The relative cancer risk peaked among patients aged 20–39 years (SIR 4.9, 95% CI 4.6–5.2) and during the first year after diagnosis (SIR 4.7, 95% CI 4.3–5.1). The SIRs for haematological, cervical, and lung cancers in cyclophosphamide-treated patients were higher than those for the corresponding cancers in the overall SLE population. Patients with SLE have increased cancer risk compared with the general population. Increased relative cancer risk is associated with younger age, first year post-diagnosis, and cyclophosphamide treatment.