Context: Research has suggested that use of combined estrogen and progestin hormone replacement therapy (CHRT) increases breast cancer risk and that CHRT use is more strongly associated with the risk of invasive lobular breast carcinoma than that of invasive ductal carcinoma. Lobular carcinoma is less common than ductal carcinoma but can be more difficult to diagnose because of its subtle elusive infiltrative pattern.
Objective: To evaluate trends in invasive lobular and ductal carcinoma incidence rates from 1987 through 1999, during which time use of CHRT increased in the United States.
Design: Descriptive epidemiologic study.
Setting: Nine cancer registries that participate in the Surveillance, Epidemiology, and End Results (SEER) program of the National Cancer Institute and that cover Atlanta, Ga; Detroit, Mich; San Francisco-Oakland, Calif; Seattle, Wash; and Connecticut, Hawaii, Iowa, New Mexico, and Utah.
Population: Women 30 years of age and older residing in the areas covered by the 9 SEER registries.
Main outcome measures: Proportional changes in incidence rates of invasive lobular and ductal carcinoma among women with no prior history of breast cancer.
Results: A total of 190 458 women were included in this analysis who were identified through the registries as having invasive breast cancer; 7682 of the 198 140 potentially eligible women (ie, those identified as not having in situ breast cancer) were excluded from this analysis because stage of cancer was unknown. Invasive breast cancer incidence rates adjusted for age and for SEER historic stage increased 1.04-fold (95% confidence interval [CI], 1.004-1.07) from 1987-1999 (206.7/100 000 to 214.1/100 000, age-adjusted). However, incidence rates of tumors classified as lobular increased 1.52-fold (95% CI, 1.42-1.63), and those classified as mixed ductal-lobular increased 1.96-fold (95% CI, 1.80-2.14); rates of these types combined increased 1.65-fold (95% CI, 1.55-1.78) (19.8/100 000 to 33.4/100 000, age-adjusted). In contrast, ductal carcinoma rates remained largely constant (153.8/100 000 to 155.3/100 000, age-adjusted; proportional change, 1.03 [95% CI, 0.99-1.06]). The proportion of breast cancers with a lobular component increased from 9.5% in 1987 to 15.6% in 1999.
Conclusions: Ductal carcinoma incidence rates remained essentially constant from 1987-1999 while lobular carcinoma rates increased steadily. This increase presents a clinical challenge given that lobular carcinoma is more difficult to detect than ductal carcinoma by both physical examination and mammography.