CYTOLOGICALLY  MALIGNANT SPINDLE CELL TUMOR
FINAL DIAGNOSIS: ATYPICAL FIBROXANTHOMA
Prior to the advent of immunoperoxidase studies, cases of this sort were dreaded by the dermatopathologist. The main differential diagnosis includes malignant melanoma, spindle cell squamous carcinoma, and atypical fibroxanthoma (cytologically malignant but biologically a low-grade lesion). The prevailing standard of care dictates more radical therapy for melanoma than for the other possibilities, so the correct diagnosis is important.

At this point in time, once the diagnosis of 'cytologically malignant spindle (and epithelioid sometimes) tumor of skin' is made on the H&E stained sections, and there is neither evidence of the radial growth phase of a melanoma nor evidence of squamous differentiation, one proceeds to immunoperoxidase studies. I usually obtain an S-100, a keratin (AE1/AE3), and a CD68. There are enough internal controls in a piece of skin that contains epidermis, nerves, and a few histiocytes that one can evaluate the preservation of the antigens under study. Although some squamous cell carcinomas may lose the keratin markers and be mistaken for atypical fibroxanthomas, this is of little consequence in that the tumor needs to be completely excised whether it is a squamous cell carcinoma, atypical fibroxanthoma, or melanoma. If one were obsessed by considerations of economy, all one would have to do is obtain the S-100 (for melanoma); all of the other possibilities are treated the same. Less common malignancies that might be considered on an individual case basis include leiomyosarcoma and angiosarcoma.

If the immunoperoxidase studies are consistent with the diagnosis of atypical fibroxanthoma, the probability of this being the correct diagnosis is enhanced if the lesion has pushing borders, is confined to the dermis, is composed of very atypical cells, and is found in sun damaged skin that has not been radiated. If  any of these factors is absent, one should place a reservation on the diagnosis. Apparent continuity with the epidermis can be seen in AFX's as well as in the other possibilities, so this is not helpful.
 
Scan power view. The black line follows solar elastotic material that has been pushed down by the tumor.
High power view in the vicinity of the black line. The characteristic of the pushing border should be assessed on the scan power image since infiltration is seen in the high power view.
High power view near the surface.Cytologically malignant spindle and epithelioid cells are present.
High power view within the main mass of the tumor. Atypical mitoses are often found in atypical fibroxanthomas.

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