Approximately 7 percent of women in Western countries develop palpable breast cysts, but the relationship between cysts and the subsequent risk of breast cancer is controversial. Microcysts are now considered to be part of the normal involutional process of breast tissue. Larger, palpable cysts may be aberrations of the involutional process, but these cysts are no longer considered a disease. The natural history of breast cysts appears to depend on the histologic type. Type 1 cysts have apocrine epithelium and contain fluid similar to intra-cellular fluid. In type 2 cysts, the epithelial lining is flattened and attenuated, and the fluid composition is similar to plasma. Type 1 cysts are more likely to progress to large, multiple lesions and may be associated more frequently with the development of breast cancer. Dixon and colleagues studied women referred to a Scottish breast unit to assess the relationship between cysts and the risk of breast cancer.

The 1,374 women in the study were referred for investigation of palpable breast cysts between 1981 and 1987. All were examined by two physicians, underwent mammography and had fine-needle aspiration of all palpable cysts. The cyst type was classified on the basis of the biochemical composition of the fluid. Data were gathered on all significant factors related to breast disease and cancer. Cases of breast cancer developing in these women were identified through the national cancer registry, hospital discharge information and linkage to death certificate registration systems. The risk of breast cancer was calculated and compared with that of women of the same ages in Scotland in 1988.

During the follow-up period, 65 new invasive cancers developed in the women studied. The median follow-up was over nine years for women who remained cancer-free and five years for those who developed cancer. Overall, the incidence of breast cancer was 2.8 times greater in patients with palpable cysts than in the general population. The greatest excess of cancers in the study group was in younger women; with increasing age, the incidence came closer to that of the general population. The risk was greatest in the year following aspiration but was still elevated after five years. The type and number of cysts were not significantly related to the risk of developing cancer.

The authors conclude that women with palpable breast cysts are at increased risk of breast cancer, especially if the cysts develop before the woman is 45 years of age. They call for further studies to confirm their finding and raise the question of regular screening of young women with breast cysts by mammography.