Lifestyle influences on the association between pre-diagnostic hormone replacement therapy and breast cancer prognosis—Results from The Danish ‘Diet, Cancer and Health’ prospective cohort

ObjectivesThe association between pre-diagnostic hormone replacement therapy (HRT) and breast cancer specific mortality as well as potential influences from other lifestyle factors on the association was investigated.Study designFemale participants from the prospective cohort “Diet, Cancer, and Health” diagnosed with breast cancer (BC) were identified and their pre-diagnostic HRT use evaluated for association with tumour biology and breast cancer outcome in multivariate analysis.Main outcome measureBreast cancer specific mortality.ResultsOf the 1212 patients originally considered 1064 were included. Of these, 105 women died from breast cancer during a median follow-up of 6.3 years (range 0.2–14.3 years). In multivariate analyses women who used HRT at enrolment into the cohort study had 47% lower risk of dying from breast cancer as compared to women who had previously or never used HRT (adjusted HR: 0.53; 95% CI, 0.37–0.85). Pre-diagnostic HRT use was associated with smaller tumour size at the time of diagnosis and a higher frequency of receptor positive breast cancer. Paradoxically, a high pre-diagnostic intake of vitamin D supplements was associated with HRT use but also with a higher BC specific mortality (HR: 1.47; 95% CI, 1.07–2.00)ConclusionsHRT use at enrolment was associated with breast tumours of smaller size at the time of diagnosis and positive receptor status, and with a lower BC mortality. The found association between vitamin D from supplements and higher BC mortality warrants further exploration.