Atypical Fibroxanthoma

Atypical fibroxanthoma. 

This is a superficial form of pleomorphic undifferentiated sarcoma, previously known as malignant fibrous histiocytoma.  It is a lesion that is particularly seen in the elderly and often presents as a pink or purple smooth rapidly growing lesion.  It can resemble an undifferentiated squamous cell skin cancer.  Histologically it is well circumscribed and is found mainly in the upper dermis as a highly cellular tumour.

There is a mixture of three cell types;  spindle shaped cells, large polyhedral cells and multi nucleated giant cells.  An undifferentiated pleomorphic sarcoma is much deeper and has much more a typicality of the cells.  The cells in an AFX look quite abnormal anyway and this is often the most readily recognisable feature.  These lesions only have a small risk of metastasis and it is not as aggressive as its atypia would suggest. However it is imperative the pathologist sees the base of the lesion to diagnose an AFX. If deep it could easily be the more aggressive pleomorphic sarcoma and it needs wide excision and follow up radiotherapy.

This tumour often has to be excluded from other spindle cell tumours such as the spindle cell squamous cell carcinoma or melanoma.  Melanomas are usually S100 positive. The squamous cell carcinoma is usually pan keratin positive and CD10 positive.  An AFX usually stains for vimentin and is strongly positive for CD10.

Atypical Fibroxanthoma    AFX 2    Pleomorphic Sarcoma