SPINDLE CELL SQUAMOUS CELL CARCINOMA, ATYPICAL FIBROXANTHOMA SIMULANT
Metaplastic or Sarcomatoid Carcinoma
The degree of departure of the morphology of the cells in a carcinoma from the parent cell line defines the degree of differentiation. It is less generally appreciated that some of the subcellular components that we depend on for immunocytochemical marker studies may also be altered or deleted in poorly differentiated carcinomas. A case in point is the spindle cell squamous cell (sarcomatoid) carcinoma of skin. In some cases the tumor cells may become AE1/AE3 negative, and the cells may become vimentin positive. I have read of examples where the cells became CD68 (a histiocyte marker) positive, but I have not personally seen such a case.

Such modifications are interesting in terms of making a diagnosis of atypical fibroxanthoma.  That diagnosis is partly dependent on excluding the possibilities of malignant melanoma, spindle cell squamous cell carcinoma, and leiomysarcoma. The exclusionary steps include the use of immunocytochemical studies. In view of the modifications mentioned above, there probably are cases that are diagnosed as atypical fibroxanthoma that really are metaplastic or sarcomatoid squamous cell carcinomas. There is no contradiction in terms of behavior; squamous cell carcinomas related to sun damage do not have a strong propensity to metastasize.

From a practical point of view, a diagnosis of atypical fibroxanthoma that is made on a small biopsy should be accompanied by a caveat that denies the absolute exclusion of  squamous cell carcinoma. There should be a recommendation for complete excision.

This is from a rapidly growing tumor of the forehead of a 63 year old male.
 

Composite scan power view (very low magnification). This large, ulcerated, protuberant tumor has expansile, pushing margins. There is not much color contrast in the slide, and the ink dots were placed by the submitting pathologist. Almost all of the tumor  to the left of the area bounded by the ink dots is composed of cytologically malignant spindle cells. There are cells in the area bounded by the ink dots that have epithelial cytology.
Medium power view of spindle cells.
High power view of some of the spindle cells. One of many atypical mitotic figures is seen.
Medium power view from the area bounded by the ink dots.
High power view of above. The orange area in the cytoplasm of the cell that is near the center of the picture may represent keratin. Intercellular bridges are barely visible above and to the left of the orange area.
Another high power view from an area where most of the cells have epithelial characteristics. Some have vacuolated or foamy cytoplasm resembling histiocytes. Abnormal mitoses are seen.

There were areas where plump spindle cells merged morphologically with cells having epithelial characteristics.

 

Click on IPOX for the immunoperoxidase studies on this case.
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