Dermoscopy of non-pigmented clonal seborrheic keratoses on legs

Seborrheic keratoses (SK) are extremely common benign epidermal lesions in adult life. Clonal seborrheic keratosis represents an unusual histopathologic subtype of SK defined by intraepidermal nests of basaloid cells, Borst-Jadassohn phenomenon. This phenomenon has also been described in hidroacanthoma simplex (the intraepidermal form of poroma) and in Bowen’s disease. Clinical and dermoscopic diagnosis is straightforward in most cases, but clonal seborrheic keratoses still remain a challenge. We present five cases of non-pigmented clonal seborrheic keratoses located on legs of adult patients. All cases were biopsied and histologically confirmed as clonal SK. Clonal SK can occur in any body site but in our experience, are especially common on the legs of adult individuals. They usually present as slowly enlarging round or oval erythematous plaques with well-defined borders and sometimes slight superficial scale. They may simulate both non-melanocytic and melanocytic tumors, such as superficial basal cell carcinomas, intraepidermal squamous cell carcinomas, Spitz tumors, and malignant melanomas. Dermoscopy of pigmented clonal seborrheic keratoses include a globular pattern, as the one observed in melanocytic naevi (1) or bluish ovoid nests as described in basal cell carcinoma (2). Non-pigmented clonal SKs show diffuse pinkish background, white network and multiple vessels (glomerular, linear-irregular and dotted vessels) (3). In conclusion, the slow progression of these lesions orientates to a benign process; however, histologic confirmation is mandatory to confirm the diagnosis and exclude skin cancer. Bibliography: 1. Squillace L, Cappello M, Longo C, Moscarella E, Alfano R, Argenziano G. Unusual Dermoscopic Patterns of Seborrheic Keratosis. Dermatology. 2016;232(2):198-202. PubMed PMID: 26812275. 2. Zalaudek I, Ferrara G, Argenziano G. Clonal seborrheic keratosis: a dermoscopic pitfall. Arch Dermatol. 2004;140(9):1169-70. PubMed PMID: 15381565. Pubmed Central PMCID: 15381565. 3. Ramyead S, Diaz-Cano SJ, Pozo-Garcia L. Dermoscopy of clonal seborrheic keratosis. Journal of the American Academy of Dermatology. 2015 Aug;73(2):e47-9. PubMed PMID: 2618399