Mastitis and the risk of breast cancer.

BACKGROUND: Many cancers arise from sites of infection and inflammation. Results from animal studies indicate that inflammatory cells may facilitate neoplastic processes by orchestrating the tumor microenvironment. Little is known about the role of inflammation in the etiology of breast cancer. The aim of this study was to examine possible associations between a history of mastitis requiring hospitalization and subsequent risk of breast cancer.

METHODS: This cohort study of 2,577,565 women used data from several Swedish population-based registers, including the Inpatient Register and the Cancer Register. The risk of breast cancer was assessed by Poisson regression modeling.

RESULTS: We identified 8411 women in the Inpatient Register with a discharge diagnosis of mastitis. Of these, 106 had a subsequent diagnosis of breast cancer recorded in the Cancer Register. Compared with women who had no recorded mastitis, the incidence rate of breast cancer (regardless of laterality) was higher in women with mastitis, with an incidence rate ratio of 1.23 (95% confidence interval [CI] = 1.02-1.49) following adjustment for age, calendar time, age at first birth and parity. In the group of women among whom information on laterality was available for both the mastitis and the malignancy (n = 87), side of lesions corresponded for 52% (95% CI = 41%-62%), which is what could be expected by chance.

CONCLUSIONS: The overall risk of breast cancer was slightly elevated in women with a history of mastitis recorded in the Inpatient Register. The absence of a correlation between laterality of lesions, however, does not support a causal association between inflammation and the development of breast cancer.

Department of Medical Epidemiology and Biostatistics, Karolinska Institutet, Stockholm, Sweden.Lambe M, Johansson AL, Altman D, Eloranta S.