Atypical fibroxanthoma (AFX) vs mimics (spindle cell melanoma, squamous cell carcinoma, etc) (video)

Atypical fibroxanthoma (AFX) vs mimics (spindle cell melanoma, squamous cell carcinoma, etc) (video)

This video from 2016 discusses the basic workup of a pleomorphic malignant cutaneous spindle cell tumor. The main differential diagnosis includes spindle cell melanoma, spindle cell squamous cell carcinoma, and atypical fibroxanthoma (AFX). Other entities in the differential include leiomyosarcoma, spindle cell angiosarcoma, and (very rarely) cutaneous rhabdomyosarcoma. A panel of S100 protein (or SOX-10), p63/p40 (and/or cytokeratins), desmin, and ERG (or CD31) can essentially exclude most of those entities, leaving only AFX. Some pathologists like to use CD68, CD10, or vimentin as positive stains for AFX. I do not personally like those stains as they are very non-specific and will stain many different tumors, including spindle cell melanoma, spindle cell squamous cell carcinoma, angiosarcoma, and many other tumors. AFX looks histologically identical to undifferentiated pleomorphic sarcoma (UPS...formerly known as MFH). The only way to distinguish between them is to determine the depth of invasion. If the tumor is confined to the dermis, then it has a very low chance of metastasis and is usually regarded as AFX. If it extends into the subcutis, then it likely has metastatic potential and should be considered as a sarcoma. For undifferentiated pleomorphic sarcomas that are confined to the dermis and subcutis on the head and neck of elderly patients, we usually use the term "pleomorphic dermal sarcoma". These may have a slightly lower metastatic potential than large deep undifferentiated pleomorphic sarcomas but can still behave aggressively.