Primary cutaneous small cell carcinoma; a case report with differential diagnosis.

Primary cutaneous small cell carcinoma (PC-SmCC) is extremely rare; only two cases have been reported in the world literatures. A 79-year-old woman presented a small cutaneous tumor in the face. Physical examination showed a tumor measuring 1.0x.08x0.6 cm in the shallow skin of the face. Excisional skin biopsy was performed. The biopsy showed complete excision of the tumor. The tumor was located in the shallow dermis and no connections to epidermis were seen. The tumor was invasive into subcutaneous tissue and surrounding dermis. The tumor was very hypercellular tumor composed of small cells with scant cytoplasm, hyperchromatic nu lei, negative nucleoli, and molded nuclei. The shapes of tumor cells are round, ovoid or spindle. The histological appearances fulfilled the criteria of SmCC of WHO. Immunohistochemically, the tumor cells were positive for cytokeratin (CK) AE1/3, CK CAM5.2, CK34BE12, CD5, CD6, CK8, p63, NSE, NCAM, synaptophysin (focal), chromogranin (focal), p53, KIT, PDGFRA and Ki-67 (labeling index (LI)=86%). They were negative for CK7, CK19, CK20, EMA, vimentin, CEA, S100 protein, CA19-9, TTF-1, MUC1, MUC2, MUC5AC and MUC6. Mucin histochemistry revealed no mucins. A molecular genetic analysis of PCR-direct sequencing identified no mutations of KIT (exons 9, 11, 13, and 17) and PDGFRA (exons 12 and 18) genes. The author diagnosed this cutaneous tumor as SmCC. Post-diagnosis whole body examination using various imaging and endoscopic techniques revealed no tumors. This may confirm that the skin tumor was primary. The cutaneous tumor was completely resected with wide margins. The patient is now followed up without therapy 8 months after the diagnosis. No recurrence or metastasis is seen. The differential diagnosis from Merkel cell carcinoma and basal cell carcinoma is very difficult and herein discussed.