Dermoscopy of clonal seborrheic keratosis

Clonal seborrheic keratoses are uncommon, are usually pigmented, and cannot be clinically differentiated from ordinary seborrheic keratoses. The histologic hallmark is the presence of sharply demarcated intraepithelial nests of basaloid cells (Borst-Jadassohn phenomenon). Although dermoscopy is a helpful diagnostic tool for atypical seborrheic keratoses, the clonal variant may represent a dermoscopic pitfall. Clonal seborrheic keratoses present with blue- brown globules and milia-like cysts on dermoscopy, but vascular structures have not been described to our knowledge.
We present an unusual acquired red patch in a young patient, showing polymorphic vessels on a diffuse pink and whitish background with white network. Polymorphic vessels on dermoscopy suggest malignancy, as they may appear in squamous cell carcinomas, Spitz nevi, and melanomas. Clonal seborrheic keratoses may present as nonpigmented patches and should be considered in the differential diagnosis of acquired erythematous vascularized lesions. Biopsy is mandatory to confirm the diagnosis.