A rare case of primary rhabdoid malignant melanoma

Case report: A 54 year old female presented with a slow growing flesh coloured nodule measuring 1 x 0.5cm on her right forearm. It was excised with 2mm margins.
Histology showed a 1.3mm thick cutaneous neoplasm composed of epithelioid cells with oval enlarged nuclei, prominent nucleoli and plentiful light eosinophilic cytoplasm. Adjacent epidermis showed an atypical junctional melanocytic proliferation. Immunohistochemistry revealed lesional cells focally immunoreactive for S100 and HMB45.
Based on the unusual morphological appearances of the melanocytes with the abundant eosinophilic cytoplasm together with the immunohistochemical profile a diagnosis of malignant rhabdoid melanoma was made. The presence of the adjacent junctional melanocytic proliferation in this case was supportive evidence of this rhabdoid melanoma being a primary rhabdoid melanoma. She was referred for wide local excision and there has been no recurrence of melanoma detected to date.
Discussion: Malignant rhabdoid tumors are characterised by sheets of large polygonal cells with abundant cytoplasm containing eosiniphilic inclusions and nuclei with prominent central nucleoli. “Rhabdoid” refers to abundant eosinophilic cytoplasm as seen in skeletal muscle tumours. The term was first used to describe an aggressive childhood renal tumour. Extra renal tumours with similar morphology have since been reported. Melanoma with rhabdoid features is an uncommon variant of malignant melanoma. Morphological features suggestive of a rhabdoid neoplasm together with positive expression for S100 protein are suggestive of a diagnosis.
Summary: Metastatic melanoma with rhabdoid features has been better described, however to our knowledge primary rhabdoid melanoma has only been reported in nine cases worldwide.

Dr. Leonard Timoney