Background: 7% of women in the western world develop palpable breast cysts. Studies of the relation between cysts and breast cancer have conflicting results. There are two clearly defined types of cyst. We investigated whether one cyst type is associated with a higher rate of breast-cancer development than the other.
Methods: We studied 1374 women with palpable breast cysts presenting between 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 fluid was less than 3, or type II if the Na+/K+ ratio was 3 or more. Data on incidence of breast cancer were available until January, 1995, and we compared them with the expected numbers of cancers calculated from age-specific breast-cancer incidence in Scotland in 1988.
Findings: 65 cancers developed during follow-up. The overall standardised incidence rate of breast cancer in patients with palpable cysts was 2.81 (95% CI 2.17-3.59). The relative incidence rate was increased for all cyst types. 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 older than 54 years had a standardised incidence rate of 1.73 (0.86-3.10). The standardised incidence rate of breast cancer was highest in the first year 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 associated relative incidence rate of breast-cancer development.