The utility of electron microscopy in dermatopathology: cutaneous SMARCA4-deficient dedifferentiated large cell neuroendocrine carcinoma

We report two cases of SMARCA4-deficient LCNEC (SD-LCNEC) presenting as cutaneous masses: an asymptomatic 3 cm right flank subcutaneous nodule in a 59-year-old male with concurrent pulmonary mass and metastases to cervical lymph node, and a painful 5 × 10 cm ulcerated left perineal mass in a 56-year-old female with hidradenitis suppurativa and concurrent pulmonary small cell carcinoma. To date, SD-LCNEC has been described almost exclusively as a thoracic malignancy. While various SMARCA4-deficient undifferentiated neoplasms (SD-UMN) have been identified in the skin, none have been definitively characterized as neuroendocrine. In both cases, neuroendocrine marker expression was markedly reduced on immunohistochemistry, precluding IHC-based classification as LCNEC. Electron microscopy performed on formalin-fixed paraffin-embedded tissue demonstrated abundant membrane-bound dense-core neurosecretory granules and well-formed desmosomes in both cases, ultrastructurally confirming the diagnosis. To our knowledge, these are the first reported cases of cutaneous SD-LCNEC confirmed by electron microscopy.