Keratoacanthoma developing in prurigo nodularis treated with cryotherapy.

We describe an elderly woman in whom keratoacanthoma developed from one nodule of prurigo nodularis that had been treated with cryotherapy for 3 months. Since in our case keratoacanthoma developed after treatment with liquid nitrogen for prurigo nodularis which had been constantly scratched in the past, we hypothesize that irritations of cryotherapy in addition to repeated mechanical traumas of scratching might have played a role in the formation of this tumor.