The one conclusive statement that can be made concerning the sum of the epidemiological evidence of a relationship between oral contraceptives and breast cancer is that there has been a remarkable lack of consistency in the findings. However, an increasing number of the recent studies suggest that there are subgroups of women who may be at increased risk of breast cancer owing to their pattern of oral contraceptive use. Of these groups, the one of most concern may be those women with long-term use beginning at a young age. The findings for use before the first full-term pregnancy or before age 25, or just for overall long duration of use are certainly suggestive and do not permit the conclusion that there is no relationship between use of the pill and breast cancer risk. Use of oral contraceptives at a young age has increased over the past 20 years, and the possible risk of breast cancer associated with early use is an important public health issue. Why would we only now be seeing an increased risk for long-term use at a young age? There are several possible explanations:
Women who use oral contraceptives are required to visit their health care provider on a regular basis to secure a renewal of their prescription. Thus, it is possible that oral contraceptive users are seen by the health care system more frequently than other women, and may receive more frequent medical surveillance, including breast exams and even mammograms. This hypothesis has led to the suggestion that women with breast cancer who use oral contraceptives are detected and diagnosed earlier than nonusers. Little is known about the natural progression of breast lesions; thus, the detection of breast cancer through mammography may indeed pick up some cases of breast cancer that might have otherwise gone undetected for many years. (This may be especially true of in situ breast cancers.) Several recent studies have incorporated into their analyses a consideration of such factors as the stage of disease, the frequency of breast self-exams and physician exams, and the frequency of mammograms as a way to account for the impact of increased surveillance (McPherson et al., 1987; Kay and Hannaford, 1988; U.K. National Case-Control Study Group, 1989). Because oral contraceptives have always required prescriptions, it seems unlikely that detection bias could be contributing enough new noise to the analyses of breast cancer and oral contraceptives to account completely for the emergence of these recent positive findings.
In recent years, the controversy about the relationship between oral contraceptive use and breast cancer risk has received much atten-