Distinctive dermal clear cell mesenchymal neoplasm: clinicopathologic analysis of five cases.

Dermal clear cell tumors are not common. This group of lesions is comprised primarily of clear cell adnexal lesions, balloon cell melanocytic lesions, and metastatic clear cell carcinomas. We report the clinicopathologic features of five cases of a novel dermal clear cell neoplasm that appears mesenchymal in nature. The affected patients included 3 men and 2 women ranging in age from 38 to 70 (median, 45 years). All the lesions occurred on the lower limb. Clinically described as smooth cutaneous nodules, size ranged from 0.5 to 2.5 cm in greatest dimension and the lesions were present from weeks to 5 years prior to excision. Situated in the reticular dermis, the tumors usually extended to involve the subcutis with sparing of the papillary dermis. The tumors were composed of large optically clear cells with vesicular nuclei. The lesions entrapped adnexal structures and thin dermal collagen fibers. Mitoses were rare (less than 1 per 25 hpf). A single case showed more pleomorphic nuclei as well as quite frequent mitoses and was considered of uncertain biologic potential. Immunohistochemistry revealed reactivity only for NKI-C3 (5/5 cases), CD68 (2/5 cases), and vimentin (2/3 cases); melanocytic, epithelial, and lymphoid markers were uniformly negative. All five lesions were locally excised; the more pleomorphic and mitotically active lesion was widely re-excised and given subsequent radiation therapy. In follow-up ranging from 1.5 to 11 years (median, 5.5 years), none of these lesions has recurred. These tumors appear to be mesenchymal in nature, but their precise line of differentiation is unknown. Recognition of these lesions is important to avoid confusion with better-known malignant neoplasms.