Desmoplastic/spindle cell squamous cell carcinoma of the skin. A diagnostically challenging tumor mimicking a scar: clinicopathologic and immunohistochemical study of 6 cases.

Desmoplastic cutaneous squamous cell carcinomas (SCCs) are rare neoplasms with an increased risk of local recurrence and metastasis usually affecting sun-exposed skin of the elderly. Microscopically, they are characterized by elongated cords of atypical epithelial cells associated with a prominent (usually reactive) desmoplastic stroma. To expand this clinicopathologic spectrum, we report 6 cases of an unusual variant of desmoplastic SCC in which the "desmoplastic" areas are predominantly composed of cytologically bland malignant spindle cells mimicking a reactive/benign scarring process. Five patients were males and a patient was a female. The median age at presentation was 72 years. Three patients had history of several years of immunosuppressive therapy for solid organ transplant. All tumors affected sun-damaged skin of the head and commonly infiltrated into the subcutaneous fat and deeper structures. Histopathologically, they were predominantly composed of relatively bland spindle cells in a fascicular pattern. Mitoses ranged from 2 to 4 per 10 high power fields. Pleomorphism was focally seen in all cases. Squamous differentiation in the invasive component was focally seen in 4 cases. SCC in situ was seen in all cases. All cases showed reactivity for keratin immunostains. Median follow-up was 19 months. Two of 6 patients died of metastatic SCC; 1 patient died of unrelated causes; and 3 patients were alive without evidence of disease. Accurate recognition of this entity is essential because of potential misdiagnosis as a benign process including scar and dermatofibroma. Careful search for atypical features and squamous differentiation, immunohistochemical studies, and, in some cases, deeper sections are required to establish the diagnosis.