Unusual malignant tumors of the breast: MRI features and pathologic correlation

Unusual malignant breast tumors are well-differentiated subtypes of invasive ductal carcinoma, including mucinous, tubular, medullary and papillary carcinomas, and account for about 10% of malignant breast tumors. They are increasingly being encountered during magnetic resonance imaging (MRI) examinations of the breast. Therefore, breast radiologists should be aware of their appearance on MRI. This review provides an overview of MRI characteristics of a range of unusual tumors (mucinous carcinoma, medullary carcinoma, tubular carcinoma, intraductal papillary carcinoma, intracystic papillary carcinoma and invasive papillary carcinoma), highlighting specific clues for diagnosis and correlating MRI and pathologic features. Many unusual breast tumors exhibit MRI features similar to those of benign or low suspicious lesions (oval shape, well-defined margins, high signal intensity on T2-weighted images, continuous increase kinetics, i.e. type I dynamic curve), leading to a possible misdiagnosis. Nevertheless, an understanding of pathologic features of these tumors, especially tissue content (mucinous, fibrous) and growth pattern, can help to define some specific clues for their diagnosis.