Postoperative/posttraumatic spindle cell nodule of the skin: the dermal analogue of nodular fasciitis.

Spindle cell proliferations of the skin are diverse, both morphologically and mechanistically. The authors have encountered four examples of a distinctive reactive/reparative cutaneous spindle cell lesion that shows homology with ones seen in the genitourinary tract and oral cavity and that is known as "postoperative/posttraumatic spindle cell nodule" (PSCN). These occurred in the skin of the face and scalp (2 cases), arm (1 case), and vulvar skin (one case), and were clearly related historically to prior episodes of trauma. The proliferations were characterized by variably-apposed and cytologically-bland spindle cells with numerous mitotic figures, set in a highly vascular stroma containing extravasated erythrocytes and inflammatory cells. All lesions were immunoreactive for vimentin, actin, and desmin, with no labeling for keratin. Postoperative/posttraumatic spindle cell nodule of the skin is a significant pseudoneoplastic lesion that is related (and perhaps identical pathogenetically) to nodular fasciitis; as such, it may be mistaken for a sarcoma or a spindle cell carcinoma. Careful attention to clinicopathologic and histologic details should result in its accurate recognition.