CD34‐positive myxoid dermatofibrohistiocytoma of the skin: an indolent post‐traumatic tumor that can be mistaken for dermatofibrosarcoma protuberans*

To the Editor, The list of skin tumors and tumor-like lesions that are CD34 positive is long and can make the task of arriving at the correct diagnosis more difficult in superficial shave biopsies. It includes many fibrous and fibrohistiocytic lesions, namely dermatofibroma (DF) and dermatofibrosarcoma protuberans (DFSPs). We describe an indolent post-traumatic dermal tumor composed of CD34-positive spindle cells and factor XIIIa-positive dendritic cells in a 48-year-old woman who presented with bilateral calf nodules, clinically diagnosed as DFs. Microscopic examination of the biopsy specimens from the right calf showed a DF. A well-circumscribed fibrohistiocytic tumor with myxoid changes located in the mid- and deep reticular dermis was identified in the left biopsy (Fig. 1). This latter lesion was unencapsulated and composed of loosely dispersed, small spindle, dendritic and histiocytic cells with a myxoid stroma (Fig. 2) that contained wispy collagen. The lesion was not associated with adnexal structures and lacked cytologic atypia, mitoses or storiform/palisaded arrangements. The tumor cells had bland oval nuclei and over 95% stained positive for CD34 (Fig. 3A). Scattered dendrocytes were factor XIIIa positive, while the spindle cells were factor XIIIa negative (Fig. 3B). Masson trichrome highlighted the meshwork of collagen bundles in the background. All cells were strongly vimentin positive, with infrequent labeling of nuclei by Ki-67 (1%). The stroma contained acid mucopolysaccharides that were stained positive for Alcian blue (pH 2.5) and negative for periodic acid-Schiff. The tumor cells were negative for S-100, epithelial membrane antigen, CD68, smooth muscle actine, alpha-1-antichymotrypsin, CD99 and FVIII-related antigen. Photomicrograph of histologic section of left calf lesion showing a relatively well circumscribed but unencapsulated myxoid fibrohistiocytic proliferation in the mid- and deep reticular dermis (hematoxylin and eosin, original magnification ×20). A) The lesion has mainly a fibrillary myxoid stroma containing wispy collagen. B) The peripheral superficial portion of the lesion showing a more spindle cell cellular component (hematoxylin and eosin, original magnification ×400). Immunohistochemical studies show A) strong CD34 immunoreactivity in the spindle cell component (endothelial cells serve as a positive control) and B) factor XIIIa decorating some dendritic cells within the lesion (immunohistochemistry, original magnification ×400). Based on the histologic features and immunohistochemical profile, our case is best classified as a CD34-positive myxoid dermatofibrohistiocytoma (DF/fibrous histiocytoma), a term coined in 1996 by Silverman and Brustein, who described an unusual subungual tumor with a similar immunohistochemical profile.1 The current tumor described herein and the one originally described in 1996 now appear to better belong to the entity described by Fetsch et al.,2 classified as superficial acral fibromyxoma (SAFM), and share many similar morphologic features. SAFM tends to involve the hands and feet, particularly the nail bed, and is composed of CD34-positive spindle/stellate cells arranged in fascicular or loose storiform growth patterns within a myxoid matrix and associated with abundant mast cells and prominent microvasculature. There is usually no atypia or increased mitotic rate. The current case did not show general morphologic features of DF such as epidermal hyperplasia, collagen trapping, sclerotic collagen or lymphohistiocytic tissue response. In addition, DF is not usually strongly CD34 positive and generally does not necessitate complete excision. Most DFSPs are CD34 positive and can be focally FXIIIa positive. However, the lesion in this case was well circumscribed and lacked the distinct storiform pattern or cartwheel spindle cell proliferation and the web-like infiltration of the underlying subcutaneous tissue typical of DFSP. The myxoid variant of DFSP is usually CD34 positive3 but may show weak or absent reactivity,4 probably because of the low cellularity in the myxoid areas. DFSP is locally aggressive and often recurs after simple excision and thus generally requires wide local re-excision. Recently, the term ‘indeterminate fibrohistiocytic lesion of the skin’ has been proposed to describe CD34-positive and factor XIIIa-positive tumors that exhibit overlapping clinical, morphologic and immunohistochemical features of DF and DFSP, and may represent a biologic spectrum between both entities.5 These lesions do not show the coexpression of both factor XIIIa and CD34 by the same cells, which may suggest the presence of two different cell populations and have the potential for local recurrence.5 Except for the presence of factor XIIIa and CD34 immunoreactivity and the lack of mitotic activity, the current case did not show the honeycomb infiltration pattern of subcutis described in all cases of indeterminate fibrohistiocytic lesion of the skin.5 Of note, there is also no recurrence 6 years after the initial presentation. Additional differential diagnoses include benign myxoid dermal lesions such as focal cutaneous mucinosis and cutaneous myxoid cyst. These lesions are not sharply defined, do not show a prominent spindle fascicular component and are only focally CD34 positive.6 Benign neural tumors may show a prominent myxoid component and may express CD347 but they are S-100 positive. Cutaneous myxoid fibroblastoma is closely related to the CD34-positive myxoid dermal dendrocytoma;8 however, these lesions are hypocellular and FXIIIa negative and contain CD34-negative multinucleated giant cells. The value of this tumor described herein is to make dermatopathologists/pathologists aware of the existence of biologically indolent (Ki-67 ≤1%) fibrohistiocytic lesions with prominent myxoid features and CD34 positivity, which could raise the possibility of myxoid variant of DFSP. It is important not to misdiagnose such lesion as a DFSP or other myxoid dermal spindle cell proliferations. Whether this proliferation is reactive, reparative or neoplastic is unknown. Interestingly, there has been no recurrence in the case described 6 years after the initial presentation. The recommended treatment for this lesion is local excision with clear margins.

Fetched live from OpenAlex and de-inverted. Abstracts are not stored in this database: the inverted indexes are 8.6 GB of the frame’s 9.3 GB of text, and the host has 13 GB free.