Unusual breast cancers: Useful clues to expanding the differential diagnosis

Invasive ductal carcinoma and ductal carcinoma in situ account for about 85% of breast cancers. Unusual breast neoplasms may be broadly divided into invasive lobular carcinoma, well-differentiated subtypes of invasive ductal carcinoma, cancers of stromal origin, and metastatic neoplasms. Clues are often present in imaging characteristics, patient demographics, and/or clinical features that may suggest that the finding is not the usual type of breast cancer. Some rare malignancies also provide specific clues to their diagnosis. This review provides an overview of unusual and a few rare malignant breast neoplasms, highlighting particular or specific clinical or imaging findings that will enable residents to expand their differential diagnosis of breast lesions beyond invasive ductal carcinoma.