Onychopapilloma Is a Nail Bed Onycholemmal Papilloma: A Clinical and Histological Study of 56 Cases, Including Seborrheic Keratosis-Like Lesions.

Onychopapilloma (OP) is a benign tumor of the nail. In all studies of the literature including the World Health Organization new classification of nail tumor, OP is proposed as a tumor originating in the distal matrix and defined histologically by its onychogenic property as "matrix metaplasia of the nail bed."The aim of this study is to clarify the origin and type of keratinization of OP, to emphasize the application of clear histological criteria according to the type of surgical procedure, to present new clinical and histological subtypes of OP, and to discuss the differential diagnosis.Eighty-six cases were available for clinicopathologic review. Detailed clinicopathological analysis could be performed on 56 cases.In addition to the classic clinical presentations of longitudinal erythronychia, longitudinal leukonychia, longitudinal melanonychia, 2 cases presented an unusual band pattern in the form of xanthonychia. Histological interpretation was more difficult for samples obtained by tangential longitudinal excision than for those obtained by longitudinal monobloc excision. The superficial eosinophilic layer of the nail bed was either clearly identified by its round to oval nuclei or showed features simulating a keratogenous zone by foci of irregularly contoured pycnotic nuclei. In all cases, the so-called matrix metaplasia of the nail bed corresponded to an onycholemmal keratinization with 2 types of cornified cells: clear cells either parakeratotic or orthokeratotic, and eosinophilic cells. The clear orthokeratotic cornified cells sometimes simulated abnormal enlarged onychocytes. The onychogenic marker including hair related keratin (HK) 31, HK85, HK86, LEF-1, and beta-catenin were not expressed in the nail bed. The samples obtained by tangential longitudinal excision frequently showed incomplete or complete deepithelialization. In incomplete deepithelialization, 2 clues suggested a diagnosis of OP: numerous filiform superficial papillae and remnants of nail bed epithelium with a superficial eosinophilic layer. In complete deepithelialization, only examination of the nail plate allowed the diagnosis to be established. Loss of the organization of the suprabasal layers, dyskeratosis, and keratinocytes with clear cytoplasm were occasionally observed. However, the absence of nuclear atypia confirmed by the normal expression of Ki67 and p53 ruled out in situ onycholemmal carcinoma/malignant OP. Two new histological subtypes have been observed. OP with lesions resembling seborrheic keratosis. OP with an overexpression of CD10 simulating the stroma of OM. Nail clipping has proven to be potentially useful in the differential diagnosis between OP with seborrheic keratosis-like lesions and onychogenic tumor such as onychocytic matricoma and onychomatricoma. In contrast to the thick nail plate perforated by cavities of onychogenic tumors, OP was characterized by a focal thin nail plate with longitudinal groove. On excised lesion, the negativity of the onychogenic markers such as LEF-1 or beta-catenin led to a definitive diagnosis of OP.OP is a nail bed onycholemmal papilloma. Numerous whorls of fusiform eosinophilic cells and true dyskeratotic cells could raise concern for the possibility of a low-grade in situ onycholemmal carcinoma. Awareness of this distinctive pattern of OP along with supplemental immunostaining will assist in enabling the correct diagnosis to be made. The KS-like variant could cause a diagnostic pitfall with onychocytic matricoma. The lack of white dots and rounded white clods on clinical examination and the focal thinning of the nail plate, on histological examination of nail clipping, provide clues for the diagnosis.