Cohort study of association of risk of breast cancer with cyst type in women with gross cystic disease of the breast.

OBJECTIVE: To assess correlation between type of breast cyst and risk of breast cancer in women with gross cystic disease of the breast. DESIGN: Cohort study of women with breast cysts aspirated between 1983 and 1993 who were followed up until December 1994 for occurrence of breast cancer. SETTING: Major cancer prevention centre. SUBJECTS: 802 women with aspirated breast cysts. MAIN OUTCOME MEASURES: Type of breast cyst based on cationic content of cyst fluid: type I (potassium:sodium ratio > 1.5), type II (potassium:sodium ratio < 1.5), or mixed (both types). Subsequent occurrence and type of breast cancer. RESULTS: After median follow up of six years (range 2-12 years) 15 cases of invasive breast cancer and two ductal carcinomas in situ were diagnosed in the cohort: 12 invasive cancers (and two carcinomas in situ) among the 417 women with type I cysts, two cancers among the 325 women with type II cysts, and one among the 60 women with mixed cysts. The incidence of breast cancer in women with type I cysts was significantly higher than that in women with type II cysts (relative risk 4.62 (95% confidence interval 1.26 to 29.7)). These results were confirmed after adjustment for several risk factors for breast cancer (relative risk 4.24 (1.12 to 27.5)). CONCLUSIONS: The increased risk of breast cancer of women with breast cysts seems to be concentrated among women with type I breast cysts.