Tumors of the nail unit. A review. Part II: acquired localized longitudinal pachyonychia and masked nail tumors.

This second part of the review categorizes the site-specific nail tumors, as proposed in the first part, according to their clinical presentations. Acquired localized longitudinal pachyonychia allows for the specific recognition of onychogenic nail tumor, which can be classified into 2 groups according to the predominant compartment of origin within the nail unit as follows: epithelial tumors encompassing onychocytic matricoma and onychocytic carcinoma, and fibroepithelial tumors: the so-called onychomatricoma. As onychomatricoma is neither an epithelial matrical tumor nor a tumor with a limited differentiation toward the matrix, the author proposes instead the descriptive term of panonychoma fibropapilliferum (POP). The designation of POP does convey to the surgical pathologist or the dermatopathologist the key morphological pattern of this tumor. It should be noted that the proposed term of panonychoma is analogous to the nomenclature that is well established for follicular neoplasms with differentiation toward all elements of the normal hair follicle, that is panfolliculoma. The second term fibropapilliferum is used to highlight the mixed fibroepithelial nature of this neoplasm, which forms multiple rudimentary nail units. These nail units construct multiple nail plates that join together and form a single thick nail plate. According to nail anatomy, 2 types of POP are described: POP of the apical matrix/eponychium, with a pseudo-condylomatous pattern, and POP of the ventral matrix with a foliated pattern in transverse sections and a fibrokeratoma-like pattern in the longitudinal sections. Suggestions for the evaluation and clinical management of localized longitudinal pachyonychia are proposed. On histology of a nail clipping, 2 patterns with clinical significance can be individualized. A horizontal alignment of large cavities indicates POP of the ventral matrix. A haphazard arrangement of smaller cavities in the nail plate, including an arrangement in the inferior two-thirds of the nail clipping, should prompt a biopsy of the distal ventral matrix to rule out a malignant lesion. In the setting of the "masked "nail tumor, a clinical subtype with some significance can be individualized, the subungual keratotic nodule growing rapidly. Three nail bed tumors are discussed within this latter group. Two new clinicopathological variants of subungual keratoacanthoma are described, and a new nail bed tumor is discussed: the infundibulocystic nail bed squamous cell carcinoma. The absence of striking nuclear atypia and the giant cystic to multicystic pattern distinguishes infundibulocystic nail bed squamous cell carcinoma from follicular infundibulocystic squamous cell carcinoma. The last section proposes a classification of folliculogenic nail bed tumors. The follicular microcysts of the nail bed have previously been called subungual epidermoid inclusions or onycholemmal cysts, but the term follicular microcysts of the nail bed is more pertinent, because of the multiple lines of follicular differentiation (infundibular, tricholemmal, apocrine, and sebaceous) seen in their benign and malignant counterparts. Absent in a large portion of the normal nail bed, the follicular microcysts seem to have a peculiar propensity for the formation of tumors that vary in maturity from simple follicular microcystic hyperplasia associated with acquired longitudinal melanonychia to microcystic nail bed hamartoma and microcystic nail bed carcinoma (the so-called onycholemmal carcinoma). The concluding tables emphasize the key and essential histological features required to make the diagnosis of site-specific nail tumors and guide appropriate therapy. The author proposes to categorize subungual tumors into 2 types: subungual skin tumors (including subungual skin metastasis from internal malignancies) and nail tumors. Nail tumors can be accurately classified using a combined clinical and histogenetic approach. This new and expanding group of appendageal tumors is important for both dermatologists and dermatopathologists for the potential early detection of a malignant lesion or for the avoidance of overtreatment of a benign lesion.