Purpose: To assess trends in invasive breast cancer and ductal carcinoma in situ (DCIS) incidence in association with changes in hormone therapy (HT) use in regular mammography screeners.
Methods: We included 2,071,814 screening mammography examinations performed between January 1997 and December 2006 on 696,385 women age 40 to 79 years; 9,586 breast cancers were diagnosed within 12 months of a screening examination. We calculated adjusted annual rates (mammogram level) for prevalent HT use, incident invasive breast cancer (overall and by tumor histology and estrogen receptor [ER] status), and incident DCIS.
Results: After a precipitous decrease in HT use in 2002, the incidence of invasive breast cancer decreased significantly in 2002 to 2006 among women age 50 to 69 years (P(trend(2002-2006)) = .005) and 70 to 79 years (P(trend(2002-2006)) = .003) but not in women age 40 to 49 years (P(trend(2002-2006)) = .45). DCIS rates significantly decreased in women age 50 to 69 years after 2002 (P(trend(2002-2006)) = .02). Invasive ductal tumors significantly declined in women age 50 to 69 years and 70 to 79 years in 2002 to 2006. In women age 50 to 69 years, invasive lobular and ER-positive cancer rates declined steadily in 2002 to 2005 (P(trend(2002-2005)) = .02 and .03, respectively), but an elevated rate in 2006 rendered the overall trend nonsignificant (P(trend(2002-2006)) = .89 and .91, respectively).
Conclusion: In parallel to the sharp decline in HT use in women undergoing regular mammography screening, invasive breast cancer rates decreased in women age 50 to 69 and 70 to 79 years after 2002, and DCIS rates decreased in women age 50 to 69 years, consistent with evidence that HT cessation reduces breast cancer risk. However, the decrease in incidence may have started to level off in 2006; this finding has not been uniformly reported in other populations, warranting further investigation.