Diagnosis: D. Schwannoma

Histology:
The core needle biopsy of the breast mass reveals a spindle cell proliferation comprised of bland cells with wavy nuclei, nuclear palisading resembling Verocay bodies, and indistinct cell borders. There is no overt epithelial component. There is no mitotic activity, no necrosis, and no cytologic atypia. An immunostain for S100 protein is diffusely positive, and immunostains for cytokeratin and CD43 are negative.

Discussion:
The histologic features of this spindle cell lesion are classic for a schwannoma, a benign peripheral nerve sheath tumor of the Schwann cells. Schwannomas can occur throughout the body and are occasionally seen on core needle biopsy of a “breast mass,” where they may be centered in the dermis, subcutaneous tissue, axillary soft tissue, chest wall soft tissue, or even in the breast parenchyma itself. The primary histologic differential diagnosis of any spindle cell proliferation of the breast on core needle biopsy is a spindle cell metaplastic carcinoma, which will display nuclear atypia, mitotic activity, and cytoerkatin immunoreactivity. In addition, the morphologic features of a schwannoma (i.e., nuclear palisading, indistinct cell borders, wavy nuclei, and hyalinized blood vessels) may be seen in variant forms of myofibroblastomas, a benign neoplasm of the breast stromal myofibroblasts. In contrast to schwannomas, myofibroblastomas are negative for S100 and variably positive for CD34, ER, desmin, and actin.

References:
1. Bellezza G, Lombardi T, Panzarola P, Sidoni A, Cavaliere A, Giansanti M. Schwannoma of the breast: a case report and review of the literature. Tumori. 2007;93(3):308-11.
2. Fisher C. Immunohistochemistry in diagnosis of soft tissue tumours. Histopathology 2011;58(7):1001-1012.
3. Magro G, Foschini MP, Eusebi V. Palisaded myofibroblastoma of the breast: a tumor closely mimicking schwannoma: Report of 2 cases. Human pathology. 2013;44(9):1941-6.