Discontinuing hormone therapy may lead to tumor regression in breast cancer, a new study indicates.

US rates of screening mammography, invasive breast cancer, and ductal carcinoma in situ (DCIS) have recently declined, noted a research team led by Diana S.M. Buist, PhD, a senior investigator at Group Health Research Institute in Seattle, Washington. Earlier research showed a rapid decline in new breast cancers as well as in the use of hormone therapy and screening mammography since 2002, when the Women’s Health Initiative announced that breast cancer rates were higher in women taking estrogen and progestin than in those taking either estrogen alone or a placebo.

Buist and colleagues sought to determine whether the rapid decline in new breast cancers could be attributed to reduced use of hormone therapy or to reduced use of screening mammography among former hormone users who are less concerned about breast cancer. After analyzing data collected from 1994 to 2009 on 163,490 women aged 50 to 79 years, all members of the Group Health Cooperative in Washington state, the investigators learned the following:

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  • Before 2002, women on estrogen and progestin therapy (EPT) had the highest screening rates (83%), followed by former EPT users (77%) and current estrogen-only therapy (ET) users (77%), former ET users (72%), and never-users (56%).
  • After 2002, screening was high among current and former EPT and ET users (approximately 81%), and significantly increased among never-users (approximately 62%).

Although DCIS rates did not change in any group, rates of invasive breast cancer dropped significantly over the whole study period for all users of hormone therapy, with the exception of current EPT users.

“Our data suggest discontinuing [hormone therapy] has an immediate effect on breast cancer rates, lending support to the mechanism that cessation leads to tumor regression,” concluded Buist’s group in Cancer Epidemiology, Biomarkers & Prevention.