This skin tumour infiltrate the dermis and efface the subcutaneous fatty tissue and is characterized by atypical cells with eosinophilic cytoplasm, round to oval often with horse-shoe nuclei and frequent multinucleated cells. The chromatin is vesicular and variable prominent nucleoli are seen. The tumour cell dissects the collagen bundles and moderate to abundant lymphoid infiltrate are seen on the background especially around the capillaries. Infrequent mitotic figures are present and proliferation index is around 15%. The neoplastic cells expressed Vimentin, CD 68, FXIIIa and strong nucleocytoplasmic positivity for S100. They are consistently negative for CD1a, CD21, CD23 and Pan Cytokeratin. Negative are also for (not shown on virtual slides) EMA, SMA, CD3, CD4, CD30, CD31, HMB45, MART-1, CK7.
Based on morphology, the main differential diagnosis of this skin tumour is between atypical fibroxanthoma (AFX), leiomyosarcoma, melanoma, and the family of histocytic&dendritic cell neoplasm. Positivity for S100 excludes AFX and negativity for cytokeratin, EMA and SMA excludes sarcomatoid squamous cell carcinoma and leiomyosarcoma. The negativity for Mart-1 and HMB45 excludes melanoma.
Taking everything into account, this skin tumour could be considered as interdigitating dendritic cell sarcoma which belongs to the family histocytic&dendritic cell neoplasm.
Dr. Tshering Dorji, MD
LAMM – IMPACTLAB Group