Family history influences the tumor characteristics and prognosis of breast cancers developing during postmenopausal hormone therapy.

Long term use of postmenopausal hormone therapy (HT) has been reported to increase breast cancer risk. On the other hand, observational studies suggest that breast cancers diagnosed during HT may have a more favorable prognosis. While family history is a risk factor for breast cancer, and genetic factors also influence prognosis, the role of family history in combination with HT use has been little studied. We investigated the relationship between HT, family history, and prognosis in 584 (267 exposed) familial and 952 (460 exposed) non-familial breast cancer cases, using three survival end points: death from breast cancer (BCS), distant disease free survival (DDFS), and local recurrence free survival (LRFS). Among non-familial cases, HT was associated with better BCS (HR 0.63, 95% CI 0.41-0.94; p = 0.025), and DDFS (HR 0.58, 95% CI 0.40-0.85; p = 0.005), with a consistent but not statistically significant effect in LRFS. This effect was not seen in familial cases (HR > 1.0), and family history was found to interact with HT in BCS (p(interaction)  = 0.0067) (BC-death) and DDFS (p(interaction)  = 0.0070). There was phenotypic heterogeneity between HT-associated tumors in familial and non-familial cases, particularly on estrogen receptor (ER) status, although the interaction between HT and family history appears to be at least partially independent of these markers (p = 0.0370 after adjustment for standard prognostic factors). If confirmed by further studies, our results suggest that family history should be taken into consideration in clinical counseling before beginning a HT regimen.