A retroperitoneal mass was detected in this 61 year old man on CT scan. CT shows a heterogenous right posterior pararenal soft tissue mass. There is a horseshoe kidney displaced anteriorly and to the left. Core biopsy suggested a poorly differentiated sarcoma. Surgical excision was performed and the final histopathology was pleomorphic liposarcoma.

80% of primary retroperitoneal tumours are malignant, the commonest tumours being lymphoma and sarcoma. The most common primary retroperitoneal sarcoma is liposarcoma. Other sarcomas which occur in this location include leiomyosarcoma, fibrosarcoma, malignant fibrous histiocytoma and rhabdomyosarcoma. Specific sarcomas tend to appear in certain age groups, for example rhabdomyosarcoma in children, and liposarcoma and malignant fibrous histiocytoma in mid- to late adult life. The peak incidence of retroperitoneal sarcoma is 40 to 60 years, with liposarcomas commoner in males but leiomyosarcomas commoner in females. Liposarcomas have 3 CT patterns based on amount and distribution of fat in tumour – solid pattern (attenuation > +20HU), mixed pattern (areas of < -20 HU and areas of > +20HU), and pseudocystic pattern (homogenous density between –20 and +20 HU). Histologically liposarcoma are classified into 5 types: well-differentiated, myxoid, round cell, pleomorphic and mixed.

1. Federle et al. Diagnostic Imaging: Abdomen. Amirsys 2004.
2. Kumar et al. Pathologic Basis of Disease. 7th edition. 2005.
Credit: Dr Bernard Ng