SOX10/keratin dual-color immunohistochemistry: An effective first-line test for the workup of epithelioid malignant neoplasms in FNA and small biopsy specimens.

BACKGROUND: The characterization of poorly differentiated neoplasms in fine-needle aspiration (FNA) and small biopsy specimens usually requires immunohistochemistry (IHC) with a panel of markers. Because of an increasing need to preserve limited diagnostic material for tumor genotyping and a mounting demand for cost containment, the authors investigated the usefulness of dual-color IHC with antibodies directed against broad-spectrum keratins and SOX10, a neuroectodermal transcription factor consistently expressed in melanoma, in the workup of epithelioid malignant neoplasms.

METHODS: A total of 107 cases of FNA cell blocks (49 cases) and small biopsies (58 cases) were selected, including 34 melanomas, 31 epithelioid/pleomorphic sarcomas, and 42 carcinomas. IHC was performed on all specimens using a peroxidase-based brown chromogen for SOX10 and an alkaline phosphatase-based red chromogen for keratins AE1/AE3. The presence or absence of staining in lesional cells was scored.

RESULTS: The majority of tumors demonstrated 1 of 3 distinct patterns: 1) malignant melanomas with nuclear SOX10 (sensitivity of 94% and specificity of 95%); 2) epithelioid/pleomorphic sarcomas negative for both SOX10 and AE1/AE3 (sensitivity of 84% and specificity of 88%); and 3) carcinomas with cytoplasmic AE1/AE3 (sensitivity of 76% and specificity of 98%). In addition, a fourth pattern with cytoplasmic AE1/AE3 and nuclear SOX10 was observed in a subset of carcinomas, most notably triple-negative breast cancers.

CONCLUSIONS: SOX10/keratin dual-color IHC appears to be an effective, sensitive, and specific test to distinguish between melanoma, sarcoma, and carcinoma. This approach can identify melanoma, prioritize additional studies, and limit the number of markers needed to workup an epithelioid malignant neoplasm, thereby potentially reducing costs and preserving valuable tissue for ancillary studies with which to guide therapy. Cancer Cytopathol 2018;126:179-89. © 2018 American Cancer Society.