Immunohistochemical distinction of cutaneous spindle cell carcinoma

Cutaneous spindle cell squamous carcinoma is an uncommon variant of squamous cell carcinoma in which keratinocytes infiltrate the dermis as single cells with elongated nuclei rather than as cohesive nests or islands, and signs of keratinization of conventional squamous cell carcinoma are insubstantial or nonexistent. Spindle cell carcinoma must be distinguished from spindle cell/desmoplastic melanoma, cutaneous leiomyosarcoma, atypical fibroxanthoma (AFX), and scar. In instances when there is no definitive evidence of squamous differentiation, immunohistochemical studies may confer diagnostic discrimination. Twenty-four cases consisting of 12 spindle cell squamous cell carcinomas, 3 AFXs, 3 leiomyosarcomas, 3 desmoplastic melanomas, and 3 scars were evaluated with a battery of immunohistochemical stains, with the specificity and sensitivity of each marker calculated. The immunohistochemical battery consisted of S-100, desmin, CD68, and smooth muscle actin and cytokeratins P KER (keratins predominantly of molecular weight 56 and 69 kd) and low-molecular weight keratin (CAM 5.2), AE1/AE3, p63, and 34 beta E12 (CK903). Spindle cell squamous carcinomas were negative for S-100, CD68, smooth muscle actin, and desmin with the exception of 2 cases with weak staining for smooth muscle actin. 34 beta E12 provided positive results for each spindle cell squamous carcinoma. The other cytokeratin stains were less sensitive for spindle cell squamous carcinoma than 34 beta E12. The final immunohistochemical results were as follows: 34 beta E12 (12/12, 100%), p63 (10/12, 80%), AE1/AE3 (8/12, 67%), low-molecular weight keratin (7/12, 58%), and P KER (4/12, 33%). The 3 AFXs were positive for CD68 and negative for all other stains, whereas the 3 leiomyosarcomas stained positively for desmin and smooth muscle actin and negatively for all other stains. The 3 melanomas stained positively for S-100 and negatively for all other immunohistochemistry. The scars were negative for all stains. In conclusion, our study of 34 beta E12 proved most promising in distinguishing spindle cell squamous carcinoma from the histologic mimickers, AFX, spindle cell melanoma, scar, and leiomyosarcoma.