HealthDay News — Long-term hormone therapy with estrogen only or combined therapy with estrogen plus progesterone is associated with a higher risk for breast cancer, study results suggest.

Ongoing use of combined hormone therapy for 10 years or longer doubled the risk for breast cancer, researchers reported at the annual meeting of the  American Association for Cancer Research in Chicago. 

The risk for cancer continued to increase with duration of treatment, and the effects of hormone therapy on cancer risk appeared strongest among women with receptor-positive tumors and those who were thinner.

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“Even though we saw an increased risk in developing breast cancer, we did not see an increased risk for dying from breast cancer,” Wendy Y. Chen, MD, MPH, from the Dana-Farber Cancer Institute in Boston, said during a press conference.

She and colleagues analyzed Nurses’ Health Study (NHS) data from 121,700 female registered nurses aged 30 to 55 at entry. They focused on a subgroup of participants who were postmenopausal during the 1980 to 2008 period, comprised of 31,959 NHS participants in 1980 that increased to 61,473 in 2008. The primary outcome was occurrence of invasive breast cancer.

During 1.57 million person-years of follow-up, 5,631 NHS participants developed breast cancer and 884 participants died of breast cancer.

Breast cancer risk increased with the duration of current estrogen plus progesteron use without evidence for a plateau. At 10 to 14.9 years, the risk ratio (RR) for breast cancer was 1.88, increasing to a RR of 2.35 for 15 to 19.9 years of use, the researchers found.

Long-term current estrogen only therapy also correlated with increased risk (RR=1.22 for 10 to 14.9 years; RR= 1.43 for 15 to 19.9 years).

There was no increase in the risk of fatal breast cancer with either combined estrogen therapy or estrogen-only use (RR=1.05,95% CI: 0.67-1.68; and RR=1.08, 95% CI=0.80 to 1.46), respectively, for 10 to 19.9 years of current use.

The investigators also examined hormone therapy use and breast cancer risk by BMI. For both combined therapy and estrogen-only therapy, current long-term use significantly increased the risk for breast cancer.

Among women with a BMI less than 25, current use of combined hormone therapy was associated with a RR of 1.79 to 3.22 versus nonusers (P<0.0001). For women with a BMI ≥25, a significantly increased risk emerged with 5 to 9.9 years of use (RR =0.45) and increased to 2.43 for 15 to 19.9 years of treatment (P<0.0001).

Current estrogen use was associated with a breast cancer risk of 1.54 to 2.00 among women with a BMI <25 (P<0.0001). Although obesity has previously been linked to an increased breast cancer risk, estrogen use among NHS participants with a BMI ≥25 was not associated with an increased risk.

Chen WY et al. “Long-term use of hormone therapy and breast cancer incidence and mortality” AACR 2012; Abstract LB-60.