Abstract

We present a diagnostic conundrum involving a pleomorphic sarcomatoid malignancy on the occipital scalp of an 81-year-old man. The initial shave biopsy findings resembled atypical fibroxanthoma, and the subsequent excision showed a large tumor with tumoral necrosis, perineural invasion, and subcutaneous extension. Focally, some tumor cells in the excision showed SOX10 expression; however, the remainder of the tumor cells in both samples were negative for S100, Melan-A, pan-cytokeratin, p63, CD31, ERG, and desmin. While pleomorphic dermal sarcoma (PDS) was considered, focal SOX10 expression raised the possibility of sarcomatoid (dedifferentiated) melanoma. Additional staining revealed diffuse PRAME expression and focal tyrosinase expression in some tumor cells and an absence of BRAF V600E and NRAS Q61R staining. Next generation sequencing revealed oncogenic mutations involving NRAS Q61L, TERT, and TP53 genes. Ultimately, the diagnosis of de-differentiated melanoma was favored given the NRAS Q61L mutation and SOX10 immunostaining. At follow-up, the patient developed metastases to the lung, liver, soft tissue and adrenal gland, supporting the impression of melanoma. The patient died of disease 23 months after initial biopsy. This case illustrates the benefit of carefully evaluating all tumor residuum when confronted with partial samples of an undifferentiated sarcomatoid malignancy on the scalp. Furthermore, it suggests a role for sequencing of such lesions when diagnostically ambiguous findings are present. Even focal expression of a melanocytic marker should cast doubt on a diagnosis of PDS and raise the possibility of de-differentiated melanoma, since therapy and outcomes are significantly different.