Acquired digital fibrokeratoma is an exophytic tumor with hyperkeratotic epidermis on acral sites. Meanwhile, cellular digital fibroma (CDF) is a unique subset of acquired digital fibrokeratomas that comprise slender spindle-shaped CD34-positive cells1. Here, we report a rare case of CDF and its clinicopathologic characteristics.

A 38-year-old Korean man presented with a painless nodule, which began to grow six months prior to examination, on the ventral part of the proximal phalanx of his right index finger. Physical examination revealed a fixed erythematous protruding nodule 0.5 cm in diameter (Fig. 1A). The results of punch biopsy for complete removal showed a polypoid tumor constricted at its base and lateral sides by acanthotic epidermis with collarette-like changes (Fig. 1B). Collagenous bundles were filled mostly with myxoid materials and numerous spindle cells arranged in a loose fascicular pattern (Fig. 1C). No nuclear atypia or mitoses were identified. Immunohistochemistry revealed most proliferative spindle cells were positive for CD34 (Fig. 1D) and vimentin (Fig. 1E) but negative for CD99, S-100, and smooth muscle actin. Prominent myxoid stroma was detected by Alcian blue stain (Fig. 1F). On the basis of these findings, the lesion was diagnosed as CDF. After the lesion was completely removed, the patient did not experience any recurrence over 18 months of follow-up.

Since McNiff et al.1 first described CDF in 2005, their observations have been regarded as important for the diagnosis of fibrohistiocytic neoplasms, which can be easily misdiagnosed as superficial acral fibromyxoma (SAFM)2. We initially considered a diagnosis of SAFM in our case; however, clinically, SAFMs are slow-growing asymptomatic tumors that commonly present as solitary lesions ranging from 0.5 to 5 cm in diameter34. The lesion in the present case was relatively smaller than typical SAFMs. Histopathologically, both SAFM and CDF can be characterized by spindle to stellate cells arranged in a storiform or fascicular pattern with variable degrees of myxoid background stroma5. Accentuated microvasculature and conspicuous mast cells are potentially suggestive of SAFM; meanwhile, keratin horn and epidermal collarette are commonly observed in CDF45. In our case, the nodule showed proliferation of spindle cells in a loose fascicular pattern; however, it was constricted at its base and lateral sides by collarette-like acanthotic epidermis. No vascular proliferation, atypical mitoses, or fibroblasts oriented vertically to the axis of the lesion were observed. In addition, the lesion was negative for CD99. Therefore, the tumor was consistent with a diagnosis of CDF. If lesions resemble myxoid fibrohistiocytic tumors (Table 1), immunohistochemistry can be used for an accurate diagnosis of CDF.

McNiff et al.1 report that the natural course of CDF appears to be benign owing it its small size, unremarkable cytology, and absence of recurrence after biopsy. However, elucidating the detailed pathophysiology and clinical characteristics of CDF requires additional case reports. Rare cases of CDF, such as the one presented herein, must be distinguished from other cutaneous fibrohistiocytic tumors including SAFM.