Postmenopausal hormone therapy and the risk of breast cancer: the view of an epidemiologist.
OBJECTIVE
Postmenopausal hormone therapy (PMH) has been widely used by menopausal women living in western countries for the past several decades. Numerous studies have evaluated the relationship between PMH and breast cancer risk because steroid hormones have been implicated in breast cancer etiology.


METHODS
A review of selected studies was performed to evaluate the history of investigations of the association between PMH and breast cancer, with a focus on studies evaluating different PMH regimens and different histologic types of breast cancer.


RESULTS
Though studies conducted before the early 1990s suggest that both combined estrogen and progestin (E + P) PMH and unopposed estrogen (E) PMH are associated with an increased risk of breast cancer, more recent observational studies suggest that E + P, particularly current use for 5 years or longer, is more strongly associated with breast cancer risk than is unopposed E. Results from the Women's Health Initiative (WHI) randomized trials have confirmed these findings as they indicate that E + P is causally related to breast cancer (relative risk (RR) = 1.24; 95% confidence interval (CI): 1.01-1.54), while E alone is not (RR = 0.77; 95% CI: 0.59-1.01).


CONCLUSIONS
There is clear and consistent evidence that use of E + P increases a woman's risk of breast cancer. Alternatively, current evidence suggests that use of unopposed E is not as strongly associated with breast cancer risk. Further studies are needed though to examine how different PMH regimens, doses, and methods of delivery are related to breast cancer risk, and how PMH impacts the risks of different types of breast cancer.
Li
Christopher I
CI
Fred Hutchinson Cancer Research Center, University of Washington, 1100 Fairview Avenue North, M4-C308, Seattle, WA 98109-1024, USA. cili@fhcrc.org
D001943:Breast Neoplasms / Q000139:chemically induced* / Q000453:epidemiology*; D015914:Estrogen Replacement Therapy / Q000009:adverse effects*; D005260:Female; D006801:Humans; D017698:Postmenopause; D016032:Randomized Controlled Trials as Topic; D014481:United States / Q000453:epidemiology