Porokeratosis and malignant skin tumors.

A survey of the past 20 years of Japanese literature revealed 198 porokeratosis patients, 23 of whom developed squamous cell carcinomas, Bowen's disease (squamous cell carcinoma in situ), and/or basal cell carcinomas on the porokeratosis skin lesions (11.6%). Malignant transformation was not found in the 55 patients of actinic-type porokeratosis. Fifty-six malignant-skin-tumor-associated porokeratosis patients, including the above 23 and those reported in other literature, were further analyzed. Thirty-nine percent of the patients developed multiple tumors. Malignant transformation often occurred after skin lesions were treated with ionizing radiation therapy. Large or coalescing skin lesions were frequently a source of malignancy development. The average age of patients when malignant skin tumors developed was 60 years; the average latency period was 36.7 years. Linear-type porokeratosis, where porokeratosis skin lesions develop at an early age, had a much longer latency period of 48.9 years, while the localized type, which develops skin lesions later and has a higher malignant transformation rate, had a shorter latency period of 22.2 years. Our results have confirmed the cancer-prone nature of porokeratosis and clarified that actinic damage does not accelerate the malignant transformation of porokeratosis skin lesions while ionizing radiation probably does. The clinical characteristics of malignant-skintumor-associated porokeratosis may provide us with useful information to help our understanding of this cancer-prone genodermatosis.