Fibroblastic connective tissue nevus: a rare cutaneous lesion analyzed in a series of 25 cases.

Fibroblastic connective tissue nevus (FCTN) represents a rare and distinct benign cutaneous mesenchymal lesion of fibroblastic/myofibroblastic lineage, which broadens the spectrum of lesions presently recognized as connective tissue nevus. A series of 25 cases of FCTN has been analyzed to further characterize the clinicopathologic spectrum and immunohistochemical features of this entity. Sixteen patients were female (64%) and 9 were male (36%), with age at presentation ranging from 1.5 months to 58 years (median, 10 y). Most patients presented with a solitary, slowly growing, painless plaque-like or nodular skin lesion. Eleven cases (44%) arose on the trunk, 9 (36%) on the head and neck, and 5 (20%) on the limbs. The lesion was present for a median duration of 11.5 months (mean, 13.2 mo). Grossly, the lesions were tan-brown to tan-white, smooth, and firm. Their size ranged from 0.3 to 2.0 cm in greatest dimension (mean size, 0.67 cm; median, 0.6 cm). All tumors showed poor circumscription and were situated primarily in the reticular deep dermis, extending into the superficial subcutis in 13 cases (52%). The lesion was associated with papillomatous epidermis in 17 cases (70%) and the presence of adipose tissue in the reticular dermis in 14 cases (60.9%). All tumors were composed of a proliferation of bland intradermal fibroblastic/myofibroblastic cells with indistinct palely eosinophilic cytoplasm and tapering nuclei, with no significant cytologic atypia or pleomorphism, arranged in short-intersecting fascicles and entrapping appendages. No mitoses were identified. Immunostains showed positivity for CD34 in 20 of 23 cases (87%) and weak focal positivity for smooth muscle actin in 9 of 19 cases (47%). No case stained positively for desmin or S100 protein. Clinical follow-up was obtained for 14 patients (median duration, 4 y). No tumor recurred locally, even when surgical excision was incomplete. No lesion metastasized. FCTN occurs most commonly as a plaque on the trunk and head/neck of children, involves deep dermis and superficial subcutis, and stains mainly for CD34. FCTN most likely represents a localized developmental dermal anomaly; it is entirely benign and should not be confused with dermatofibrosarcoma protuberans or other neoplasms such as dermatomyofibroma.