Clinical history: 58 year old male with rapidly growing mass on right thigh. The lesion was clinically in keeping with a sarcoma. Excision was performed.
What is your diagnosis?
Discussion and answer
Merkel Cell Carcinoma
Microscopic Description: Sections show a tumour in the deep dermis and subcutaneous fibroadipose tissue. The tumour is composed of cords and nests of atypical cells with round, mildly pleomorphic nuclei, and scant amount of cytoplasm. Nuclear moulding is present. The chromatin is granular with small nucleoli. The mitotic rate is brisk and there are focal areas of necrosis.
Immunohistochemical studies show the malignant cells are positive for cytokeratin 20, keratin AE1:AE3, keratin CAM5.2, epithelial membrane antigen, chromogranin, synaptophysin and neuronal specific enolase. The keratins show a paranuclear dot-like pattern. The tumour cells are negative for TTF-1, cytokeratin 7, desmin, smooth muscle actin, S-100 protein, HMB-45 and CD99. The histologic features and the immunohistochemical profile are consistent with Merkel cell carcinoma.