Adult-onset reticulohistiocytoma presenting as a solitary asymptomatic red knee nodule: report and review of clinical presentations and immunohistochemistry staining features of reticulohistiocytosis

Abstract

Reticulohistiocytomas are benign dermal tumors that usually present as either solitary or multiple, cutaneous nodules. Reticulohistiocytosis can present as solitary or generalized skin tumors or cutaneous lesions with systemic involvement and are potentially associated with internal malignancy. A woman with a solitary red nodule on her knee is described in whom the clinical differential diagnosis included dermatofibroma and amelanotic malignant melanoma. Hematoxylin and eosin staining and immunoperoxidase studies of the biopsy specimen established the diagnosis of adult-onset reticulohistiocytoma (solitary epithelioid histiocytoma). Reticulohistiocytoma is characterized by mononuclear, and occasionally multinuclear, histiocytes with eosinophilic "glassy" cytoplasm. The immunohistochemical profile of a reticulohistiocytoma demonstrates consistent positive expression for CD68 (a marker that is expressed by histiocytes but can also show positive staining in melanomas and carcinomas), CD163 (a very specific marker for histiocytes), and vimentin. Reticulohistiocytomas show variable positive expression for MITF (microphthalmia transcription factor) and S100 protein, both of which are more commonly used as markers for melanocytes. Recurrence of a reticulohistiocytoma is rare, even for patients with an incompletely removed lesion. However, our patient elected to have her residual tumor completely excised.

The Pervasive Forager-Solitary Reticulohistiocytoma

“…Diffuse histiocytic infiltration is admixed with lymphocytes and dermal fibrosis. Typically, histiocytic cells are incorporated with a dense, pink cytoplasm designated as "oncocytic" or "ground glass" cytoplasm [5,6]. Solitary reticulohistiocytoma is constituted by enlarged, mononuclear and multinucleated histiocytic cells incorporated with abundant, eosinophilic "ground glass" cytoplasm [5,6].…”

“…Morphological assessment demonstrates a monomorphic infiltrate of epithelioid histiocytic cells extending throughout the dermis. An infiltrate of lymphocytes is admixed with the histiocytic cells [5,6]. Lesions are constituted of mononuclear and multinucleated histiocytes imbued with abundant, finely granular, amphophilic cytoplasm with a characteristic "ground-glass" appearance [5,6].…”

“…An infiltrate of lymphocytes is admixed with the histiocytic cells [5,6]. Lesions are constituted of mononuclear and multinucleated histiocytes imbued with abundant, finely granular, amphophilic cytoplasm with a characteristic "ground-glass" appearance [5,6]. Proliferating histiocytes are encompassed by an admixture of diverse inflammatory cells such as granulocytes and T lymphocytes although a population of B lymphocytes is absent [5,6].…”

“…Few histiocytes demonstrate vacuolated cytoplasm. Lymphocytic infiltrate is exceptionally discerned [5,6]. Lesions can depict a folliculo-centric pattern of vertical cellular expansion with an absence of alteration of superimposed stratified squamous epithelium with extension into the subcutaneous adipose tissue.…”

“…Lesions can depict a folliculo-centric pattern of vertical cellular expansion with an absence of alteration of superimposed stratified squamous epithelium with extension into the subcutaneous adipose tissue. Thus, trauma-induced lesions of solitary reticulohistiocytoma can demonstrate an atypical cellular growth pattern [5,6].…”