A case of malignant hidroacanthoma simplex

Malignant transformation of hidroacanthoma simplex into in situ porocarcinoma, also known as hidroacanthoma simplex with invasive growth, is very rare. Malignant hidroacanthoma simplex generally arises in the elderly and occurs most frequently on the lower extremities and trunk. The gross appearance of the tumor tends to vary, and there are nodular lesions or keratotic plaques with or without erosion and/or ulceration. In some cases, it clinically resembles Bowen's disease or seborrheic keratosis. Histopathologically, the tumor appears similar to hidroacanthoma simplex, but the cells are atypical with hyperchromatic nuclei, and many are in mitosis. Here we report an extremely rare case of malignant hidroacanthoma simplex that occurred on the right knee of an 84-year-old Korean woman.