E1868. Primary Retroperitoneal Masses: CT and MR Imaging Findings Review
  1. Susana Lopes Rodrigues; Instituto Português de Oncologia Francisco Gentil
  2. João Santos; Instituto Português de Oncologia Francisco Gentil
  3. Hálio Rodrigues Duarte; Instituto Português de Oncologia Francisco Gentil
Primary retroperitoneal masses are rare tumors that arise within the retroperitoneum and outside the major organs. They form a heterogeneous group of tumors that can be either benign or malignant (most common). Determining whether the tumor is located within the retroperitoneal space should be the first step in its characterization, but sometimes it can be difficult. Knowledge of specific patterns of spread, tumor components and tumor vascularity is useful and it can help to narrow the differential diagnosis. As the treatment options vary, radiologists should be able to differentiate these masses by using imaging criteria.

Educational Goals / Teaching Points
To provide a systematic approach of primary retroperitoneal masses based on cross-sectional imaging appearance and to describe the differential diagnosis.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
Computed tomography (CT) is the most common tool used for characterization of retroperitoneal masses. Magnetic resonance (MR) may be used for further characterization due to its superior soft-tissue contrast. MR may be useful to assess vascular invasion and to evaluate the fat content of lesion. Specific signs as the “beak sign,” the “embedded organ sign,” the “phantom organ sign” are useful for identifying the organ of origin which is an important step to characterize these retroperitoneal tumors. However, diagnosis may be challenging and histologic biopsy is always mandatory.

CT and MR imaging play an important role in characterization of these lesions, in the assessment of the extent of the disease and involvement of adjacent structures and distant disease. Familiarity with the CT and MR imaging features of various retroperitoneal masses and with specific diagnostic clues is essential in making an accurate diagnosis of primary retroperitoneal tumors and in obtaining clinically significant information.