Rare Case of Squamous Cell Carcinoma Arising from an Intraosseous Epidermal Cyst: A Diagnostic Challenge

We report a rare case of squamous cell carcinoma (SCC) arising from an intraosseous epidermal cyst (EC) in the distal phalanx of the left thumb. A 76-year-old male presented with progressive thumb pain experienced over the previous six months. Radiography revealed a radiolucent lesion without marginal sclerosis, and magnetic resonance imaging showed peripheral contrast enhancement with no solid components. Surgery revealed a bone-originating mass without adhesion to the surrounding skin or nail bed, which histopathological findings determined contained both cystic epithelium with laminated keratin and invasive keratinizing tumor cells, confirming SCC arising from an intraosseous EC. No primary lesion or lymph node enlargement was identified by postoperative computed tomography. Although wide resection and chemotherapy were proposed, the patient declined further intervention beyond the curettage performed during surgery, opting for close observation only. No recurrence or metastasis has been observed in the five years since the surgery. Intraosseous ECs are extremely rare, with malignant transformation even more uncommon. Accurate diagnosis requires histopathological confirmation, as imaging alone is insufficient. This case highlights the importance of considering intraosseous EC in the differential diagnosis of bone lesions and underscores the need for further case accumulation to clarify optimal management strategies.