Abstract

Dermatofibromas, or fibrous histiocytomas, are common and encompass a large number of variants. An exceedingly rare subtype has recently been defined as adenodermatofibroma. There have only been ten cases described in the literature. Adenodermatofibromas are characterized by dilated glandular structures within a dermal proliferation of spindle-shaped fibroblasts and histiocytes. Additionally, there is often a prominent vascular proliferation. Here we describe an additional case and discuss the differential diagnosis. A 45 year-old man presented with a dome-shaped, pink-tan nodule on the flank. On exam, he was found to have a positive dimple sign with a tract to an underlying fluid-filled sac. The clinical differential diagnosis was concerning for dermatofibroma, dermatofibrosarcoma protuberans, and other cystic neoplasms. Excisional biopsy showed epidermal hyperplasia and a cellular proliferation in the dermis. There were numerous small spindled to round cells and clusters of histiocytes with entrapped collagen fibers around the periphery, which was strongly suggestive of dermatofibroma. In addition, there were prominent and slightly dilated capillaries throughout the lesion. However, complicating the diagnosis was the presence of a cystic glandular structure lined by cuboidal to columnar epithelium with apocrine features. Immunohistochemical staining confirmed the diagnosis of adenodermatofibroma with the spindle cells showing strong positivity for CD68 and factor XIIIa, and negativity for CD34. Although the differential diagnosis would include collision tumors, apocrine adenoma/hidrocystoma, and vascular neoplasms, the constellation of histologic features is consistent with adenodermatofibroma. We present this case to make others aware of this unusual entity.