RISK OF BREAST-CANCER IN WOMEN WITH BREAST CYSTS

Objective: To study the occurrence of breast cancer in women with breast cysts. Design: Prospective follow-up study. Setting: Office surgical practice. Patients: All 742 women referred to the practice with breast cysts diagnosed by means of aspiration or, occasionally, biopsy between 1969 and 1985. Main outcome measures: The incidence of breast cancer and the number of years between diagnosis of breast cyst and diagnosis of cancer. The observed number of cases of breast cancer was compared with the expected number, calculated from Ontario rates of breast cancer. Results: Fifteen of the women died but did not have breast cancer. No follow-up information was available for five women. Another 38 were lost to follow-up; they did not have breast cancer at the last contact, after 2 to 17 years of follow-up. These patients were withdrawn from the study in the year in which they died or were last observed. By 1990, 34 (5%) of the women had breast cancer. The overall ratio of observed:expected cases of cancer was 3.04 (95% confidence interval 2.09 to 4.28). Breast cancer developed after 7.5 years, but the average length of follow-up was only 10.1 years. Only 3.8% of 374 women after 10 years and 5.4% of 141 women after 15 years had breast cancer. Conclusion: Women who have a gross breast cyst are at moderately increased risk of breast cancer, which usually develops only after many years.