Background: Malignant melanoma is an aggressive malignancy that can present as a poorly differentiated neoplasm. Loss of S100 and melanA antigenicity can make pathologic identification difficult, especially in those patients who lack a cutaneous primary lesion. Immunostaining with SOX10, a key nuclear transcription factor in the differentiation of neural crest progenitor cells to melanocytes, has a high reported sensitivity and specificity for pathologic identification of melanoma in difficult cases.
Case Report : A 69-year-old male with a history of heavy tobacco use presented to the otolaryngology clinic with a left parotid mass. He underwent a parotid gland biopsy, which was significant for a high grade, poorly differentiated malignancy of unclear primary source. A staging PET/CT demonstrated localized hypermetabolic activity in the draining left cervical lymph node basins. He underwent a left modified radical neck dissection and parotidectomy. Pathologic assessment demonstrated a 3.9 × 1.6 × 1.6 cm3 poorly differentiated carcinoma with perineural invasion and 8/85 lymph nodes involved. Morphologically, it had features of a high grade epithelioid tumor with spindle cell features. Immunohistochemical (IHC) stains were negative for epithelial markers (AE1/3, EMA, CK5/6, CAM5.2), smooth muscle actin, CD34, S100, and melanA. Given the concern for a spindle cell melanoma that lost its antigenicity for S100 and melanA, a SOX10 IHC stain was performed.
The SOX10 immunostain demonstrated strong, diffuse positivity which secured the diagnosis of malignant melanoma. Molecular testing for BRAF and KIT mutations was negative. The nal diagnosis was a stage IVA (pT2pN2bM0) malignant melanoma of the parotid gland without a cutaneous primary lesion. The patient received a course of adjuvant radiation to a total dose of 66Gy and will complete one year of adjuvant immunotherapy with Nivolumab.
Conclusion: Malignant melanoma can present as a poorly differentiated malignancy and may be difficult to diagnose by providers, especially in the absence of a typical clinical history and a primary cutaneous lesion. In cases where the standard melanoma immunostains are negative, IHC staining with SOX10 can help secure the diagnosis with high sensitivity and specificity.
