Track

Abstract

Sarcomatoid undifferentiated malignant melanoma (SUMM) is defined as a rare and aggressive type of malignant melanoma (MM) characterized by complete lack of all melanocytic markers. This can occur in the context of pre-existing bona-fide MM with a phenotypic switch to another histologic type, often simulating primary sarcomas referred to as dedifferentiated MM (DMM) or in the absence of MM, referred to as sarcomatoid undifferentiated MM (SUMM). Given the rarity of SUMM/DMM and overlapping histopathologic features in primary de novo high grade sarcomas, they are often misclassified as the latter, prompting a markedly different therapeutic approach. Herein, we present a case of SMM with aberrant strong and diffuse cytokeratin (CK) expression mimicking primary lung carcinoma with cutaneous and soft tissue metastases. A 56-year-old female presented with a right lung mass and a left buttock mass. Histomorphology from buttock mass revealed a superficial and relatively circumscribed dermal-based neoplasm comprised of sheets of epithelioid and spindle cell with large pleomorphic nuclei, coarse chromatin, prominent nucleoli and eosinophilic cytoplasm. The tumour cells were positive for AE1/AE3 and CK8. The lung mass was given the clinical consideration of lung primary and the histopathology mirrored those of the cutaneous lesion, defining a malignant keratin-positive pleomorphic epithelioid and spindle cells neoplasm. NRASQ61R mutation was noted on both specimens, initially prompting the diagnosis of primary pleomorphic carcinoma of the lung with cutaneous metastasis. However, comprehensive genomic profiling showed high tumour mutation burden, unequivocal UV-light mutational genomic signature and other molecular features seen in SMM.