Under the Microscope: How would you diagnose this case?

A 57 year old woman with a history of colon polyps underwent a routine screening colonoscopy. A 2.2 cm polyp was discovered at the dentate line and removed. Below are two photomicrographs of the polyp. The correct diagnosis is:

a) Mucosal prolapse polyp b) Spindle cell melanoma
c) Squamous cell carcinoma, sarcomatoid type d) Angiosarcoma

Answer: Squamous cell carcinoma, sarcomatoid type

This polyp is an unusual variant of squamous cell carcinoma, known as the spindle cell or sarcomatoid variant. The polyp surface consists of a layer of squamous epithelium, which is severely dysplastic. The bulk of the polyp consists of a spindle cell proliferation beneath this layer of epithelium. The spindle cells are relatively small but exhibit a high nuclear:cytoplasm ratio and are very mitotically active; these features are compatible with a malignant process. In some areas they appear to form rudimentary anastomosing vascular channels, suggesting a possible angiosarcoma; in other areas the cells have eosinophilic cytoplasm, which is suggestive of rhabdomyosarcoma. In scattered foci, the dysplastic squamous epithelium dipped down and blended into the spindle cell proliferation in a seamless fashion.

The differential diagnosis includes a spindle cell squamous cell carcinoma and a sarcoma with incidental overlying squamous dysplasia. In workup of the tumor, the most important aspect is to determine the nature of the spindle cell proliferation. An immunohistochemical stain for CD31 would be immunoreactive in a vascular tumor such as an angiosarcoma; this stain was negative. An immunostain for desmin was negative, eliminating the possibility of a rhabdomyosarcoma. The possibility of a spindle cell melanoma, which do occur at this site, was ruled out by a negative S100 immunostain. A p63 immunostain is used to confirm a squamous cell origin of a tumor. In this case, the p63 stain was immmunoreactive in the dysplastic squamous epithelium as well as in the underlying spindle cell proliferation, indicating that the spindle cell component represents a poorly differentiated invasive squamous cell carcinoma.

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