Dermatofibroma with myxoid changes in a patient with psoriasis.

A 38-year-old female with psoriatic arthritis developed a dermatofibroma (DF) on her upper arm. Its position was not exactly on a psoriatic plaque; however, psoriatic lesions were present diffusely around the DF lesion. Histological examination revealed the typical features of DF with myxoid changes in the portion between the tumor nest and the overlying epidermis. The mast cell number was significantly increased over that of solitary DFs without myxomatous lesions. It was suggested that mast cells may play a role in induction of the myxoid changes in the DF lesion in this case.