Pathologic Features of Malignancies Presenting as Asymmetry on Mammography.

The majority of breast cancers have a solid tumor growth pattern and are seen on mammography as dense masses with defined borders. Cancers detected as asymmetry are rare, and little has been published about their pathologic features. These cancers do not form discrete masses, and a border is not evident. This retrospective case series was undertaken to identify malignancies presenting as asymmetry, to describe their histologic and biologic features and to correlate these features with the mammographic appearance. During the 7.5 years of the study, 18,419 coreneedle biopsies were performed and 42 cases of malignancy presenting as asymmetry were diagnosed (0.2%). The majority were invasive carcinomas (30% or 71%), followed by ductal carcinoma in situ (9% or 21%) and lymphoma (3% or 7%). The invasive carcinomas could be divided into 3 groups: very small unifocal (T1a) carcinomas, larger unifocal carcinomas, and cases with multiple foci of invasion. The latter group had a higher rate of lymph node metastases and more stage III cancers. The invasive carcinomas were predominantly of special histologic types and associated with a minimal stromal response. In contrast, the cases of ductal carcinoma in situ tended to be of higher grade and elicited periductal fibrosis, which likely contributed to the increased density seen on mammography. Although most of the invasive carcinomas were of favorable biologic type (97%) and were stage I (67%), triple-negative carcinomas and stage III carcinomas were also detected. When evaluating core needle biopsies performed for asymmetry, pathologists should be aware that these cancers can have a subtle infiltrative appearance with little or no desmoplastic response, mirroring their appearance by imaging.