Lifetime recreational exercise activity and risk of breast carcinoma in situ.

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The incidence rates of breast carcinoma in situ (BCIS) have increased dramatically over the past two decades, primarily because of increased mammography screening. Ductal carcinoma in situ, which accounts for approximately 85% of BCIS and 10-20% of all breast carcinomas, is generally recognized as the final step in the progression to invasive disease. To the authors' knowledge, few studies have been conducted to date to evaluate BCIS risk factors. Because of its potential effects on circulating sex hormones, physical activity has been proposed as a modifiable risk factor for invasive breast carcinoma. However, the relation to BCIS risk is poorly understood. The authors analyzed data from a population-based case-control study conducted in Los Angeles County. Personal interviews were conducted with 567 white and black women (age range, 35-64 years) who had been newly diagnosed with BCIS between March 1, 1995 and May 31, 1998 and with 1026 control subjects, of whom 616 were screened within 2 years of identification. After excluding unscreened control subjects (n = 410) and adjusting for potential confounding factors, the risk of BCIS was approximately 35% lower among women with any exercise activity compared with inactive women, although no significant trend was observed. The association between exercise activity and the risk of BCIS was modified by a family history of breast carcinoma. No reduction in risk was observed among women reporting a first-degree family history of breast carcinoma (homogeneity of trends P value = 0.02). The findings of the current study suggest that exercise activity may modify the risk of BCIS, particularly among women without a family history of breast carcinoma.