Giant Cell Tumor of the Distal Ulna With Contiguous Involvement of the Distal Radius: A Report of a Rare Case.

Giant cell tumor (GCT) of bone is a benign but locally aggressive neoplasm that commonly affects skeletally mature individuals. While GCT most commonly affects the distal femur, proximal tibia, and distal radius, involvement of the distal ulna is uncommon. Contiguous extension of a distal ulna GCT to the adjacent distal radius is exceedingly uncommon and presents unique diagnostic and surgical challenges. We report a 23-year-old right-hand-dominant male who presented with a four-month history of progressive pain and swelling of the distal forearm. Radiographs demonstrated an eccentric, expansile lytic lesion involving the distal ulna with cortical thinning. Magnetic resonance imaging (MRI) revealed a subarticular lytic lesion with cortical breach and surrounding marrow edema. An image-guided core needle biopsy confirmed GCT. The lesion was classified as Campanacci Grade III. The patient underwent en bloc resection of the distal ulna. Intraoperatively, a small lytic lesion was identified on the adjacent ulnar surface of the distal radius and excised with wide margins. Histopathological examination confirmed identical GCT features in both specimens, establishing contiguous tumor extension rather than a separate primary lesion. At the 12-month follow-up, the patient demonstrated full wrist range of motion, good grip strength, and no evidence of local recurrence. This case highlights that GCT of the distal ulna can rarely extend contiguously to adjacent bones. Meticulous intraoperative inspection of all adjacent bone surfaces is essential to achieve complete oncological clearance and optimize functional outcomes.