Disseminated Superficial Actinic Porokeratosis in a Patient with Psoriasis, after Long-Term Narrowband Ultraviolet B Phototherapy

Porokeratosis is a rare epidermal disorder characterized by annular or linear hyperkeratotic plaques with slightly raised thread-like borders, and in most cases, atrophic centers. Disseminated superficial porokeratosis and disseminated superficial actinic porokeratosis (DSAP), which primarily involve sun-exposed areas, are common types of porokeratoses. Histologically, a column of parakeratotic cells, a so-called cornoid lamella, is a hallmark of porokeratosis. Porokeratosis is considered to result from the inability to eliminate an abnormal keratinocyte clone induced by genetic factors and various stimuli, including sunlight, artificial ultraviolet light, viral infections, immunosuppressive conditions (hematologic malignancies, organ transplants, or autoimmune disease), and immunosuppressive therapies. Here, we report a 59-year-old Korean woman with DSAP that developed after narrowband ultraviolet B (NB-UVB) therapy for psoriasis. Our case emphasizes the occurrence of DSAP due to NB-UVB that is able to induce local immunosuppression at the irradiated site; the pathogenesis of DSAP remains unclear.