Proposed hypothesis and rationale for association between mastitis and breast cancer.

Breast cancer is amongst the most common forms of cancer, is predominantly a woman's illness, and is the most frequently reported invasive cancer in women worldwide (Bray et al., 2018). Varying risk factors have been identified, including genetics, family history, lifestyle, age and the use of hormone replacement therapy. Mastitis, also predominantly a woman's illness, is an inflammatory condition of the breast that, despite being an inflammation-related condition, is not currently considered a risk factor for breast cancer. This appears counterintuitive as epidemiological studies have identified chronic inflammation as a contributor to cancer risk, for example in gastric, oesophageal and colon cancers (Lin et al., 2016; Qadri et al., 2014; Principe et al., 2017). Previous reports have focused on women hospitalised for mastitis, and most commonly on puerperal mastitis, perhaps underestimating the relationship between breast cancer and non-lactational mastitis. Our hypothesis, based on systematic review, suggests that a longitudinal study of this disease, affecting women predominantly, is warranted.