Retroperitoneal sarcomas are rare neoplasms that arise within the retroperitoneal tissue but outside the organs of retroperitoneum. Sarcomas arise from mesenchymal stem cells in muscle, fat and connective tissue . Majority of soft tissue sarcomas arise in limb or the limb girdle, only 10% to 20% of sarcomas arise in retroperitoneum . Common histologic subtypes of retroperitoneal sarcomas are liposarcoma (41%), leiomyosarcoma (28%), malignant fibrous histiocytoma (7%), fibrosarcoma (6%) and malignant peripheral nerve sheath tumors (3%) [1,2}.
Most soft tissue sarcomas are sporadic, few have associated conditions or found to have identifiable causes. Sarcoma may develop following radiotherapy for lymphoma, cervical cancer, testicular tumors and breast cancer. Genetic disorders associated with soft tissue sarcomas such as neurofibromatosis type 1 which has a 10% risk of developing a malignancy in peripheral nerve sheath. Genetic type of neuroblastoma has a high risk of developing osteosarcoma and soft tissue sarcomas. Chronic lymphedema – associated angiosarcoma (Stewart -Treves syndrome) can occur as a rare complication following treatment for breast cancer .
Retroperitoneal sarcomas commonly present as an abdominal mass without other symptoms. The peak incidence is 5th decade of life although they can present at any age and has an equal sex incidence [1,2]. Imaging plays a major role in evaluating retroperitoneal sarcomas, to depict precise localization of tumor, determine the extension, invasion and characterization of specific pathologic type.
CT is the most useful imaging modality used to evaluate retroperitoneal tumors. Several sings may indicate the tumor origin within the retroperitoneum but outside of retroperitoneal organs. Demonstration of several features will help identify the tumor origin. Anterior displacements of retroperitoneal organs suggest the lesion arise within the retroperitoneum. When the mass deforms the adjacent edge of an organ to a beak shape it is likely the mass origin from that organ (the beak sign) . When the tumor compress and forms a crescent shape deformity of the adjoining organ it is considered the mass arise outside that organ. A part of tumor embedded in an organ usually indicates that the tumor arise from that organ (embedded organ sign) . Hypervascular retroperitoneal masses are supplied by prominent arteries that can be identified by CT or MRI which may be helpful in identifying its origin [3}.
CT and MRI occasionally suggest a histological diagnosis. Presence of fat within a large retroperitoneal mass favors the diagnosis of liposarcoma. A retroperitoneal mass with calcification with extensive hemorrhage without fat component or central necrosis favors diagnosis of malignant fibrous histiocytoma. Although the cross sectional imaging cannot accurately predict the stage visualization of tumor necrosis indicate high grade tumor and poor prognosis . Whole body FDG PET performed in combination with CT provides metabolic information of the tumor. It is also reliable in differentiating high grade sarcoma from low grade lesions depending on the biological activity. PET scan also helps in identifying malignant lymphadenopathy, distant metastases and detecting local recurrent disease .
Primary retroperitoneal sarcomas have a high rate or recurrence after local resection even when the surgical margins were negative for tumor. Follow up with imaging as the patients are usually asymptomatic and clinically detected only in 50% [5,6].