Acral fibromyxoma with loss of Rb1 by immunohistochemistry and fluorescence in situ hybridization: A diagnostically exploitable marker

RB1 loss in the mesenchymal component of onychomatricoma

BACKGROUND

Onychomatricoma is a nail neoplasm that usually presents as longitudinal nail plate thickening, involving either the partial or whole nail. Histopathologically, it is characterized by deep invaginations of the proliferating nail matrix and proliferation of CD34+ and CD10+ spindle cells with collagenous to myxoid stroma. Onychomatricoma has been considered a fibroepithelial neoplasm. Recently, RB1 loss has been verified using array comparative genomic hybridization.

METHODS

This study investigated the RB1 status in onychomatricoma with morphological methods.

RESULTS

Six patients with onychomatricoma were included in the study. RB1 status was assessed using immunohistochemical staining and fluorescence in situ hybridization. Immunohistochemical staining showed that all six cases experienced RB1 loss in the mesenchymal component of onychomatricoma but not in the proliferated nail matrix. Fluorescence in situ hybridization in five cases showed a monoallelic deletion of the RB1 locus in the mesenchymal component but not in the proliferated nail matrix.

CONCLUSIONS

RB1 loss was observed only in the mesenchymal component of onychomatricoma. Our findings suggest that the proliferated nail matrix in onychomatricoma represents reactive hyperplasia of various degrees secondary to neoplastic mesenchymal proliferation. This indicates that onychomatricoma should be recognized as an RB1-deleted soft tissue neoplasm rather than a fibroepithelial neoplasm.

Application of immunohistochemical studies in diagnosing emerging superficial mesenchymal neoplasms

Molecular diagnostics, with the subsequent development of novel immunohistochemical markers, continues to advance and expand the field of soft tissue pathology. As such, the ever-evolving molecular diagnostic landscape will continue to shape and refine our understanding and classification of neoplasms. This article reviews the current literature on various tumors of mesenchymal origin, including fibroblastic/fibrohistiocytic, adipocytic, vascular, and tumors of uncertain origin. We aim to give the reader a detailed understanding and pragmatic approach to various new and established immunohistochemical stains in diagnosing these neoplasms and also discuss various pitfalls with significant repercussions.

An unusual growth in the nail matrix: A case of superficial acral fibromyxoma

We describe a case of superficial acral fibromyxoma arising within the germinal matrix of the index finger. This is an uncommon localisation of this relatively newly described benign soft tissue tumour. Herein, we discuss the varied clinical presentation, distinguishing histopathological features and important differential diagnoses for this condition.