Diagnostic Approach to Mesenchymal and Spindle Cell Tumors of the Breast.

Mesenchymal and spindle cell tumors of the breast represent a broad and heterogeneous group of lesions that may be sampled on core needle biopsy or surgical excision. Mesenchymal lesions unique to the breast are those that derive from the specialized breast myofibroblast, such as mammary myofibroblastoma and pseudoangiomatous stromal hyperplasia. However, any mesenchymal lesion arising in extramammary soft tissue may also arise in the breast, including fibroblastic, peripheral nerve sheath, adipocytic, and vascular lesions. The spindle cell lesions pose the greatest diagnostic challenge, due to the significant radiographic, morphologic, and immunophenotypic overlap within the category of mesenchymal lesions and more broadly with other nonmesenchymal breast lesions. The distinction is particularly challenging on the limited material of breast core needle biopsies, and caution should be taken before definitively classifying a breast spindle cell lesion on core needle biopsy to avoid unnecessary treatment if misdiagnosed. Consideration of a wide differential diagnosis, adequate sampling of a resection specimen, use of a targeted immunopanel, and selective use of molecular assays are essential steps for accurate classification of mesenchymal lesions in the breast. This review covers the clinical, histologic, and immunophenotypic features of mesenchymal tumors of the breast, with a special emphasis on the differential diagnoses unique to the breast and challenges encountered on breast core needle biopsy.