Risk of Newly Incident Diabetes Mellitus and Treatment Risk Factors in Breast Cancer Survivors: Landmark Analyses of Nationwide Data

Background While breast cancer increases the risk of diabetes mellitus (DM), its temporal relationships according to age group and treatment risk factors for DM have not been comprehensively investigated. In this study we explore temporal patterns of DM risk in breast cancer survivors, stratified by age and risk factors. Methods Using the National Health Insurance Service database in South Korea, this retrospective cohort study analyzed 65,982 breast cancer survivors and 168,214 age-matched controls, excluding those with prior DM or other cancers. Multivariable Fine-Gray models adjusted for sociodemographics, comorbidities, lifestyle behaviors, and cancer treatments assessed DM risk, with landmark analyses at 1, 3, and 5 years post-diagnosis and stratification by age (≤ 50 and > 50 years). Results In women ≤ 50, DM risk was highest in the first year post-diagnosis (sHR 3.74, 95% CI 3.08–4.55), decreased at 1 year (sHR 1.11, 95% CI 1.03–1.19), and showed no significant increase in 3-year and subsequent analyses. In women > 50, DM risk was also elevated in the first year (sHR 1.71, 95% CI 1.92–2.35) but not later. Significant risk factors included BMI ≥ 25 (sHR 1.45), smoking (sHR 1.72), hypertension (sHR 1.48), dyslipidemia (sHR 1.49), and taxane use (sHR 1.15). Tamoxifen was a risk factor in younger women (sHR 1.22, 95% CI 1.06–1.40). Conclusion Breast cancer survivors face the highest DM risk within the first year post-diagnosis, particularly younger women. Risk factors include obesity, smoking, hypertension, dyslipidemia, and treatments such as taxane and tamoxifen.