A Case of Cystic Retroperitoneal Dedifferentiated Liposarcoma Diagnosed by Percutaneous Image-Guided Biopsy

Introduction: Dedifferentiated liposarcoma (DDLP) was initially defined as a tumor containing differentiated liposarcoma and distinct regions of nonlipogenic spindle cell or pleomorphic sarcoma. Retroperitoneal liposarcomas feature a characteristic appearance with a predominantly fatty component, and cystic liposarcomas are rare. We describe a case of retroperitoneal DDLP predominantly consisting of multilocular cysts. Case Presentation: A 77-year-old man previously visited a doctor because an echo scan unexpectedly revealed an abdominal tumor. Contrast computed tomography (CT) disclosed a large multilocular cystic tumor spanning from the left upper abdomen to the retroperitoneum, and poorly marginated soft tissue structures were present around the abdominal aorta, inferior vena cava, pancreas, mesentery, and left kidney. CT also revealed a right lung mass. The soft tissue structures in the retroperitoneal cystic tumor and right lung mass were strongly enhanced on 2-deoxy-2-[fluorine-18] fluoro-<sc>d</sc>-glucose positron emission tomography, suggesting a malignant retroperitoneal tumor and lung metastasis. CT-guided percutaneous biopsy targeting the left perirenal soft tissue structure was performed, and the tumor was diagnosed as DDLP. Lung metastasis was present, and the retroperitoneal tumor surrounded multiple organs. Therefore, the tumor was not suitable for surgical resection but it was indicated for chemotherapy based on multidisciplinary discussion. Conclusion: We experienced a case of retroperitoneal cystic DDLP diagnosed by percutaneous image-guided biopsy and treated appropriately based on the pathological diagnosis.