Multiple carcinoma in patients with psoriasis caused by psoralen-ultraviolet A therapy, natural killer cell insufficiency, or intensified sun exposure?

A case is presented of a patient with psoriasis in whom squamous cell carcinoma most likely occurred due to the interaction of various factors (psoralen-ultraviolet A therapy, sun exposure, and suppression of natural killer cell function). Therapy consisted of 1 mg/kg/daily of etretinate (Tegison), which resulted in clearing of psoriatic lesions in one month's time. New carcinomas did not develop and natural killer function improved.