Risk of breast cancer in women with palpable breast cysts: a prospective study

Background. 7% of women in the western world develop palpable breast cysts. Studies of the relation between cysts and breast cancer have conflicting r esults. There are two clearly defined types of cyst. We investigated whethe r one cyst type is associated with a higher rate of breast-cancer developme nt than the other. Methods. We studied 1374 women with palpable breast cysts presenting betwee n 1981 and 1987, who had cysts aspirated between 1981 and 1989. Cysts were classified as type I if the sodium/potassium (Na'/K') ratio in the cyst flu id was less than 3, or type II if the Na+/K+ ratio was 3 or more. Data on i ncidence of breast cancer were available until January, 1995, and we compar ed them with the expected numbers of cancers calculated from age-specific b reast-cancer incidence in Scotland in 1988. Findings. 65 cancers developed during follow-up. The overall standardised i ncidence rate of breast cancer in patients with palpable cysts was 2.81 (95 % Cl 2.17-3.59). The relative incidence rate was increased for all cyst typ es. The standardised incidence rate of developing breast cancer among women younger than 45 years was highest at 5.94 (2.97-10.63), with a significant trend for decreasing relative incidence rate with age (p < 0.05). Women ol der than 54 years had a standardised incidence rate of 1.73 (0.86-3.10). Th e standardised incidence rate of breast cancer was highest in the first yea r after aspiration (7.02 [3.73-12.00]) but the risk was still raised after 5 years (2.68 [1.84-3.76]). Interpretation. Women with breast cysts are at an increased risk of breast cancer, especially at younger ages. The type of cyst did not alter the asso ciated relative incidence rate of breast-cancer development.