First Case Report of a Huge Giant Cell Tumor of Soft Tissue Originating from the Retroperitoneum.

Objective: Rare disease Background: Giant cell tumor of soft tissue (GCT-ST) is a rare disease generally generated from superficial tissue. We report an extremely rare case of giant cell tumor of soft tissue arising from retroperitoneal tissue. Case Report: A 78-year-old man visited our medical center with the chief complaint of fatigue and weight loss for 1 month. He had a hard and immobilized mass without pain in the left upper quadrant. Contrast-enhanced CT showed a huge tumor (22x20x16 cm) in the retroperitoneal space, and it invaded into the stomach, colon, pancreas, spleen, and left kidney. MRI demonstrated the tumor had a serous cystic component as T1 was low, T2 was high, and DWI was slightly high. We diagnosed the retroperitoneal malignant tumor, and tumor resection was performed with total gastrectomy, partial colectomy, distal pancreatectomy, left nephrectomy, and left adrenalectomy for complete resection, without any postoperative complications. The tumor predominantly consisted of a solid mass, and had necrotic lesions, cystic lesions, and calcification. The histological exam showed it was composed of spindle and multinucleated giant cells; however, there was no cellular atypia or pleomorphism. Immunohistochemical staining characterized the tumor with CD68+, SMA+, CD34-, Desmin-, and S-100-. We finally diagnosed it as GCT-ST with the intermediate group of malignancy, according to WHO criteria. Thereafter, the patient had no recurrence at 1 year after resection. Conclusions: The huge GCT-ST arising from the retroperitoneal space, which has never before been reported, was successfully resected. We report it with pathological findings to add to the relevant literature.