Underlying breast cancer risk and menopausal hormone therapy

The recent Collaborative Group on Hormonal Factors in Breast Cancer (CGHFBC) publication calculatedthe attributable risk of breast cancer from use of estrogen alone and estrogen plus a syntheticprogestogen for less than 5 to 15 or more years of use. This CGHFB report calculated attributablerisk based on their findings of relative risk from pooled data from 58 studies. Notably, neither theCGHFBC nor other previous studies have examined the effect of underlying risk of breast cancer onattributable risk. This omission prompted us to determine the magnitude of the effect of underlying riskon attributable risk in this perspective. Meaningful communication of the potential risk of menopausalhormonal therapy requires providing women with the estimated risk above their existing underlyingrisk (ie, attributable risk). Therefore, we have estimated attributable risks from the data published bythe CGHFBC, taking into account varying degrees of underlying risk. Based on the Endocrine SocietyGuideline on Menopausal Hormone Therapy (MHT), we divided groups into 3 categories of risk: low(1.5%), intermediate (3.0%), and high (6.0%) underlying risk of breast cancer over 5 years. In womentaking estrogen plus a synthetic progestogen for 5 to 9 years, the attributable risks of MHT increasedfrom 12, to 42, to 85 additional women per 1000 in the low-, intermediate-, and high-risk groups,respectively. The attributable risks for estrogen alone were lower but also increased based on underlyingrisk. Notably, the attributable risks were amplified with duration of MHT use, which increased bothrelative risk and breast cancer incidence.