MALIGNANT PROLIFERATING ONYCHOLEMMAL CYST

A slowly growing malignant tumor of the nail unit in a 74-year-old fem ale is reported. At light microscopy, the tumor was composed of small keratinous cysts with abrupt central keratinization, and of solid nest s and strands of atypical keratinocytes filling the dermis and penetra ting the phalangeal bone. In our view, this can be regarded as the mal ignant analog of the keratin cysts arising from the nail bed epitheliu m, as first reported by Samman in 1959. Since, according to several au thors, the nail bed epithelium is comparable to the outer root sheath of the follicle and since the reported tumor showed some analogies wit h malignant proliferative trichilemmal cyst, it is suggested that this new entity be designated as malignant proliferating onycholemmal cyst .