Soft Tissue, Vascular, and Neural Tumors plus Diagnostic Techniques
Vascular tumors
Hemangiomas show well-formed vessels with bland endothelium. Angiosarcoma shows dissecting atypical endothelial channels and hobnail cells, with CD31, ERG, and CD34 positivity; radiation-associated cases show MYC amplification. Kaposi sarcoma shows slit-like spaces, the promontory sign, and HHV8 positivity. Epithelioid hemangioendothelioma shows cords in myxohyaline stroma with a WWTR1-CAMTA1 fusion.
Neural and soft tissue tumors
Schwannoma shows Antoni A and B areas, Verocay bodies, and strong S100 positivity. Neurofibroma shows wavy cells, patchy S100, and CD34 positivity. Perineurioma is EMA positive and S100 negative. Malignant peripheral nerve sheath tumor shows loss of H3K27me3 and patchy S100. Dermatofibroma is factor XIIIa positive, CD34 negative, with collagen trapping. Dermatofibrosarcoma protuberans is storiform, CD34 positive, traps fat, and harbors COL1A1-PDGFB. Atypical fibroxanthoma and pleomorphic dermal sarcoma show atypical spindle cells in sun-damaged skin; deep invasion and necrosis favor sarcoma.
Diagnostic techniques
Immunohistochemistry panels must be interpreted in context, since no single marker is definitive. Direct immunofluorescence requires proper transport in Michel or Zeus solution, and patterns are diagnostic: intercellular, linear basement membrane, or granular. Special stains include PAS for fungi and basement membrane, GMS for fungi, Fite for mycobacteria, and mucin stains for Paget disease. Molecular tests include T-cell receptor clonality, MYC amplification, and fusion testing for EWSR1, COL1A1-PDGFB, and WWTR1-CAMTA1.
Traps
CD34 is not specific for dermatofibrosarcoma protuberans, since solitary fibrous tumor, spindle cell lipoma, and neurofibroma also stain. A negative S100 does not exclude neural tumor. Never interpret a clonal molecular result without compatible morphology.
Rule of thumb
Storiform with CD34 and fat trapping means dermatofibrosarcoma protuberans; storiform with factor XIIIa and collagen trapping means dermatofibroma. A shoulder plaque with storiform spindle cells infiltrating fat and strong CD34 positivity is dermatofibrosarcoma protuberans, confirmed by COL1A1-PDGFB fusion and treated with Mohs.