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Abstract

The diagnosis of spindle cell/sarcomatoid squamous cell carcinoma is often challenging to make, particularly in small biopsy specimens. Reasons include a lack of a definite in situ of more differentiated invasive component, as well as limited reactivity for cytokeratins and/or p40. Less commonly, the presence of a spindle cell tumor adjacent to a conventional squamous cell carcinoma suggests the possibility of a collision tumor. We present a case of an 82 year old male with a history of diffuse large B cell lymphoma and non-Hodgkin lymphoma who presented to clinic with innumerable skin lesions, including a left mid scalp lesion that was diagnosed as “spindle cell squamous cell carcinoma” at an outside hospital. Repeat biopsy of the lesion showed a polypoid proliferation of markedly atypical spindled and epithelioid cells, adjacent to a well-to-moderately differentiated invasive squamous cell carcinoma with an associated in situ component. These cells were negative for multiple broad-spectrum cytokeratins (pancytokeratin, CAM 5.2, 34betaE12), p40, SOX10, SMA, and ERG. Based on these findings, the differential included a terminally dedifferentiated squamous cell carcinoma and a collision tumor of squamous cell carcinoma and atypical fibroxanthoma. On excision, the tumor had a similar morphology to the atypical fibroxanthoma-like area seen on biopsy. However, tumor cells demonstrated patchy positivity for p40, consistent with a spindle cell/sarcomatoid squamous cell carcinoma. This case highlights a less common, but important potential pitfall in the diagnosis of malignant spindle cell tumors on sun-damaged skin.