To illustrate the main radiological signs that help to establish the retroperitoneal (RP) origin of the tumor; A pictorial guide with key patterns of some of the most common solid primary RP tumors; To provide a diagnostic algorithm and narrow the differential diagnosis.

Solid primary retroperitoneal masses represent a rare, but broad spectrum of lesions that arise within the retroperitoneal space, but outside the major organs in this space. Cross-sectional imaging with computer tomography (CT) and magnetic resonance imaging (MRI) play an important role in demonstrating key features of these tumors.

Anatomy of the Retroperitoneal SpaceThe retroperitoneum is a compartmentalized extraperitoneal space that extends from the diaphragm superiorly to the pelvis inferiorly, and is situated between the posterior parietal peritoneum anteriorly and the transversal fascia posteriorly.It is divided into 3 compartments by well-defined fascial planes: anterior renal fascia (Gerota’s fascia), posterior renal fascia (Zuckerkandl’s fascia), and lateroconal fascia (Fig. 1).[Fig 1]Compartments Anterior Pararenal Space Limits: Anterior: parietal peritoneum Posterior: anterior renal fascia (Gerota) Laterally: lateroconal fascia. Content: ascending and descending colon, duodenum, pancreas. Perirenal Space Limits:...

Solid retroperitoneal tumors are rare and represent diverse groups of lesions with overlapping and non-specific imaging findings; A systematic approach to such lesions is essential and implies knowledge of retroperitoneal anatomy and characteristic imaging findings, such as the composition, enhancement pattern, location, spread, and growth pattern, combined with clinical information.

C-G. Pitusac-Prisacaru: Nothing to disclose C. Lupascu-Ursulescu: Nothing to disclose L. Gheorghe: Nothing to disclose C-E. Hincu: Nothing to disclose D. Negru: Nothing to disclose

Kanemura, T. et al. “Understanding Retroperitoneal Anatomy for Lateral Approach Spine Surgery.”Spine Surgery and Related Research1 (2017): 107 - 120. Gulati V. et al. “Solid Primary Retroperitoneal Masses in Adults: An Imaging Approach”. Indian J Radiol Imaging. 2022 Jul 13;32(2): 235-252. Porrello, G et al. CT and MR Imaging of Retroperitoneal Sarcomas: A Practical Guide for the Radiologist. Cancers 2023, 15, 2985. Yacoub JH, Clark JA, Paal EE, Manning MA. Approach to Cystic Lesions in the Abdomen and Pelvis, with Radiologic-Pathologic Correlation. Radiographics. 2021 Sep-Oct;41(5):1368-1386....