Affiliations of authors: K. A. Rosenblatt, University of Illinois at Urbana-Champaign, Champaign, and Fred Hutchinson Cancer Research Center, Seattle, WA; D. L. Gao, Station for Prevention and Treatment of Cancer, Shanghai Textile Industry Bureau, China; R. M. Ray, D. B. Thomas, Fred Hutchinson Cancer Research Center.
Correspondence to: Karin Rosenblatt, Ph.D., Department of Community Health, 120 Huff Hall, University of Illinois at Urbana-Champaign, 1206 S. Fourth St., Champaign IL 61820 (e-mail: krosenbl{at}uiuc.edu).
Lipworth et al. (1) presented a comprehensive review of the published literature on breast-feeding and breast cancer in a recent issue of the Journal. Evidence that prolonged lactation is associated with a reduced risk of breast cancer comes solely from casecontrol studies and has not been consistently observed. The cohort studies that were cited in the review were performed in countries (United States and Norway) where few women breast-fed for a long period of time. Here, we present preliminary results from a cohort study in Shanghai, where the prevalence of prolonged breast-feeding is common.
The Shanghai Textile Workers' cohort study is imbedded in a randomized trial of breast self-examination (2). The trial included current and retired employees associated with 520 factories in the Shanghai Textile Industry Bureau. Enrollees in the trial responded to an orally administered baseline questionnaire, from October 1989 to October 1991, which ascertained information on reproductive history, breast-feeding practices, contraceptive use, and history of gynecologic surgeries. Ascertainment of breast cancer cases that occured in the cohort until December 30, 1998, was accomplished through the review of medical records and records from factories, the Shanghai Textile Industry Bureau Cancer Registry, and the Shanghai Cancer Registry. In 2078315 person-years of follow-up, 1176 incident cases of breast cancer were identified. Analyses were performed among parous women with the use of Cox proportional hazards models, and rate ratios (RRs) in relation to breast-feeding were adjusted for age (with the use of linear splines), parity, and age at first birth.
A significant reduction in risk was not observed in women who had ever breast-fed (RR = 0.97; 95% confidence interval [CI] = 0.821.16); however, in women who had breast-fed for longer than 36 months (see Fig. 1), a significant reduction in risk was observed (RR [3748 months] = 0.70 [95% CI = 0.490.98]; RR [49+ months] = 0.64 [95% CI = 0.450.92]). The reduction in risk was restricted to postmenopausal women in our cohort, but this reduction may have been due to the fact that premenopausal women generally had fewer children (due to China's recent family planning policy) and had shorter total months of lactation.
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REFERENCES
1
Lipworth L, Bailey LR, Trichopoulos D. History of breast-feeding in relation to breast cancer risk: a review of the epidemiologic literature. J Natl Cancer Inst 2000;92:30212.
2
Thomas DB, Gao DL, Self SG, Allison CJ, Tao Y, Mahloch J, et al. Randomized trial of breast self-examination in Shanghai: methodology and preliminary results. J Natl Cancer Inst 1997;89:35565.
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