Breast cancer extent and survival among diabetic women in a Finnish nationwide cohort study

Breast cancer (BC) and diabetes mellitus (DM) are major health problems. We examined the association between DM and BC stage at diagnosis and subsequent survival in a Finnish cohort of female BC patients. All BC cases (N = 73,170) diagnosed in 1995–2013 with dates and causes of death were identified from the Finnish Cancer Registry. Participation in organized mammography screening was obtained from Mass Inspection Registry. Information on DM diagnoses and background conditions recorded during 1995–2013 were obtained from national Care Register for Health Care and merged to data on medication use from the national Prescription Register. Logistic regression with adjustment for mammography screening and age at BC diagnosis was used to evaluate the risk of advanced stage BC at diagnosis. Cox regression was used to evaluate overall and BC survival. Analyses were adjusted for age, background conditions and mammography screening. Survival analyses were further adjusted for tumor extent, histology and primary treatment. Of the cohort 11,676 (16.0%) had DM. Screening participation did not differ by diabetes. Compared to non-diabetic women, diabetics had more often locally advanced (odds ratio, OR 1.26; 95% CI 1.18–1.35) or metastatic BC (OR 1.59; 95% CI 1.44–1.75) at diagnosis. During a median follow-up of 5.8 years after BC diagnosis 10,900 (14.9%) women died of BC. Risk of BC death was higher among diabetic compared to non-diabetic women (HR 1.36; 95% CI 1.27–1.46). Risk of BC death increased with duration of DM. This supports DM as a risk factor for fatal BC.