Supplementary Material for: A Case of Cellular Digital Fibroma on the Lateral Aspect of the Left Foot: a case report

Introduction: Cellular digital fibroma (CDF) is a rare, benign CD34-positive fibrous tumor that typically arises on acral sites, most commonly the fingers and toes. Clinically, it may resemble acquired digital fibrokeratoma, digital fibroma, periungual fibroma, supernumerary digit, or verruca vulgaris, and histopathologically it can be confused with other CD34-positive acral tumors, particularly superficial acral fibromyxoma (SAF). Therefore, accurate diagnosis requires careful clinicopathological and immunohistochemical evaluation. Case Presentation: A 48-year-old woman presented with an asymptomatic, well-circumscribed, keratotic nodule measuring 5 mm in diameter on the lateral margin of the left foot, an uncommon location for CDF. Histopathological examination revealed compact orthokeratosis with an epidermal collarette, and a dermal proliferation of uniformly slender spindle cells arranged in short interweaving fascicles. No cytologic atypia, mitotic figures, or myxoid stroma were observed. Immunohistochemically, the tumor cells were diffusely positive for CD34 and negative for α-smooth muscle actin, epithelial membrane antigen, S100 protein, and CD99. Based on these findings, a diagnosis of CDF was established. The lesion was completely excised, and no recurrence was observed during a 2-year follow-up period. Conclusion: Although rare, particularly at uncommon sites such as the lateral foot, CDF should be included in the differential diagnosis of small keratotic acral tumors. Awareness of its characteristic clinicopathological and immunohistochemical features is essential to distinguish CDF from both clinically similar benign lesions and other CD34-positive acral neoplasms, thereby avoiding unnecessary overtreatment.