G-CSF-producing Undifferentiated Pleomorphic Sarcoma Adjacent to the Ascending Colon and in the Right Iliopsoas Muscle: A Case Report and Review of the Literature.

Undifferentiated pleomorphic sarcoma (UPS) is a soft tissue sarcoma, occurring most commonly on the lower extremities. We herein report a rare case of primary UPS adjacent to the ascending colon and in the right iliopsoas muscle. Computed tomography of the abdomen revealed large masses, and the patient experienced a high-grade fever, leukocytosis, elevated serum C-reactive protein level, and hematopoietic activation on 18F-fluorodeoxyglucose-positron emission tomography. This inflammatory reaction was caused by granulocyte colony-stimulating factor secreted by tumor cells. Surgical resection was performed, and the inflammatory reaction disappeared immediately. The patient received adjuvant chemotherapy and survived one year after the operation without evidence of recurrence.