Benign spindle cell tumors of the mammary stroma: Diagnostic criteria, classification, and histogenesis

Purely benign mesenchymal spindle cell neoplasms of the breast are currentl y labeled under various terms in the literature (benign spindle cell tumor, fibroma, spindle cell lipoma, myofibroblastoma, solitary fibrous tumor, my ogenic stromal tumor). The lack of strict diagnostic criteria to clearly in dicate such mesenchymal neoplasms is the main reason which generated the ri sk of terming the same lesion under different names or, conversely, of coll ecting different types under the same term. Although such neoplasms exhibit morphological and immunophenotypical heterogeneity, they actually represen t variations of the same tumor entity, likely arising from the uncommitted vimentin(+)/CD34(+) fibroblasts of the mammary stroma, capable of multidire ctional mesenchymal differentiation. To cover the entire spectrum of such lesions, the term "benign spindle cell tumors (BSCTs) of the mammary stroma" is advocated. BSCTs can be subtyped into four main groups by light microscopy (LM) and immunocytochemistry (ICC ): fibroblastic, myofibroblastic, fibro-histiocytic, and mixed forms. A sim ple and practical approach to a nosologically correct diagnosis and a list of differential diagnoses are presented. The awareness of the diversity of morphological and immunophenotypical features of BSCTs of the mammary strom a, including uncommon variants, is helpful to avoid confusion with other mo nomorphic bland-looking benign and malignant spindle cell tumors and tumor- like lesions of the breast.