<p> CME Activity</p><p>Learning objectives: 1. Describe the imaging features of primary neoplastic and non-neoplastic retroperitoneal lesions. 2. Recognize specific imaging findings related to the different types of retroperitoneal tumors. 3. Discuss the differential diagnosis of primary retroperitoneal neoplasms. </p><p>Multiple-choice questions 1. Which of the following is true about primary retroperitoneal neoplasms? a- Primary retroperitoneal neoplasms are common tumors.</p><p> b- Metastasis is common in high-grade sarcomas.</p><p> c- Metastasis spread occurs through the lymphatic system.</p><p> d- Recurrence rates are low.</p><p> e- Retroperitoneal sarcomas are often very small at the time of diagnosis.</p><p>Neville A, Herts BR. CT characteristics of primary retroperitoneal neoplasms. Crit Rev Comput Tomogr 2004; 45(4):247–270.</p><p>2. What is the most frequent primary retroperitoneal tumor?</p><p> a- Liposarcoma</p><p> b- Leiomyosarcoma</p><p> c- Malignant fibrous histiocytoma</p><p> d- Rhabdomyosarcoma</p><p> e- Neuroblastoma</p><p>Rajiah P, Sinha R, Cuevas C, Dubinsky TJ, Bush WH, Kolokythas O. Imaging of Uncommon Retroperitoneal Masses. RadioGraphics 2011; 31:949–976.</p><p>3. Which of the following entities has the most FDG uptake? a. Well-differentiated liposarcoma</p><p> b. Dedifferentiated liposarcoma</p><p> c. Myxoid liposarcoma</p><p> d. Pleomorphic liposarcoma</p><p> e. Lipoma</p><p>Kitajima K, Kono A, Konishi J, Suenaga Y, Takahashi S, Sugimura K. 18F-FDG-PET/CT findings of retroperitoneal tumors: a pictorial essay. Jpn J Radiol 2013; 31:301–309.</p><p>4. Which of the following conditions manifests as a large retroperitoneal mass with necrosis and contiguous involvement of a vessel?</p><p> a- Liposarcoma</p><p> b- Leiomyosarcoma</p><p> c- Malignant fibrous histiocytoma</p><p> d- Rhabdomyosarcoma</p><p> e- Gastrointestinal stromal tumors</p><p>Rajiah P, Sinha R, Cuevas C, Dubinsky TJ, Bush WH, Kolokythas O. Imaging of Uncommon Retroperitoneal Masses. RadioGraphics 2011; 31:949–976.</p><p>5. Which of the following is true about retroperitoneal lipomas? a- Lipomas are rare in the retroperitoneum.</p><p> b- On images, lipomas can be confused with well differentiated liposacomas.</p><p> c- A fat-containing mass without enhancement or soft tissue component is seen on imaging.</p><p> d- Malignant change of lipoma to liposarcoma is virtually unknown</p><p> e- All the above. Craig WD, Fanburg-Smith JC, Henry LR, Guerrero R, Barton JH. From the Archives of the AFIP. Fat- containing Lesions of the Retroperitoneum: Radiologic-Pathologic Correlation. RadioGraphics 2009; 29:261–290.</p>