Transformation of Seborrheic Keratosis into Bowenoid Actinic Keratosis via Three Steps of Histological Change in a Patient with Rheumatoid Arthritis Treated with Multiple Immunosuppressants

We report a case of seborrheic keratosis (SK) that transformed into bowenoid actinic keratosis (AK) via three steps of histological change in a 77-year-old woman. The patient presented with a multiple-year history of a brownish lesion on the right cheek. She reported that some months earlier she had noted a pinkish lesion developing within the brownish lesion. She had also been treated with several immunosuppressants for rheumatoid arthritis for many years. Physical examination revealed a nodule measuring 13 × 12 mm on the lateral side of the right upper cheek. The lesion comprised three regions: a brownish hyperkeratotic region in the upper portion; a pinkish region in the lower portion; and a slightly dented, band-like region between the other two regions. Histopathologically, the specimen consisted of four zones: SK comprising basaloid cells; SK composed of squamoid cells; atrophic AK; and bowenoid AK. The zones of SK with basaloid cells and squamoid cells clinically corresponded to the brownish hyperkeratotic region. Atrophic and bowenoid AK zones corresponded to the dented, band-like region and pinkish region, respectively. Collectively, the nodular skin lesion was diagnosed as SK associated with atrophic and bowenoid AK within the SK lesion. The present case suggests that bowenoid AK developed from SK by malignant transformation via three steps of histological change. The facts that our patient had received treatment with several immunosuppressants and that no other AK lesions were evident around the AK support the notion that in this case, bowenoid AK developed from SK by malignant transformation.