Pure and mixed mucinous carcinoma of the breast: pathologic basis for differences in mammographic appearance

It is important to differentiate pure and mixed mucinous carcinomas of the breast because the former have a more favorable prognosis. We correlate histopathologic findings with mammographic findings for 20 patients with pure or mixed mucinous carcinomas to determine differential characteristics and their pathologic basis.

We searched our pathology database of 2219 consecutive women with a diagnosis of breast cancer and found that 31 (1.4%) had mucinous carcinomas. Eleven women were excluded because the mammograms (n = 6) or the microscopic slides (n = 5) were not available. The remaining 20 women, who were 37-92 years old (mean, 64 years), were the subjects of the study. Their clinical records, mammograms, and microscopic slides were reviewed. Tumor size was based on the maximum mammographic diameter or measured diameter of the excised tumor.

Seventeen patients (85%) had tumors that were mammographically apparent. Histopathologic review confirmed 15 pure mucinous tumors and five mixed mucinous tumors having an overall mean diameter of 3.4 cm. The pure-tumor group contained three incidentally detected tumors (all < or = 0.8 cm in diameter); six that had a circumscribed, lobular contour on mammograms (mean diameter, 3.6 cm); and six that had a poorly defined, irregular contour (mean diameter, 1.2 cm). One of the mammographically apparent small pure tumors contained histologically confirmed psammomatous microcalcifications. All pure tumors had microscopically evident circumscribed margins that could have accounted for the circumscribed mammographic appearance of the larger masses. All mixed tumors had mammographically and histologically evident irregular margins because of the associated fibrosis and infiltrative margins of the nonmucinous component (mean diameter, 5.3 cm).

There are differences in the mammographic appearances of pure and mixed mucinous carcinomas that have a histopathologic basis. Circumscribed, lobular margins on mammograms are characteristic of large pure tumors and are the result of their microscopically evident circumscribed margins and expansile growth pattern. Irregular margins on mammograms are more characteristic of mixed mucinous tumors, regardless of tumor size, and are attributable to the fibrotic and infiltrative nature of the nonmucinous component.