Retroperitoneum and Pelvic Extraperitoneum: Anatomic Landmarks, Imaging Features, and Patterns of Disease Spread.

The retroperitoneum and pelvic extraperitoneum are complex anatomic spaces that contain numerous vital structures such as major vessels, lymphatics, nerves, and abdominopelvic organs. These regions are divided by fascial planes into distinct compartments with defined boundaries and contents, which guide the spread of disease within and beyond the abdominopelvic cavity. The retroperitoneum represents the abdominal part of the extraperitoneum located posterior to the parietal peritoneum and extends from the diaphragm superiorly to the pelvic brim inferiorly. The pelvic extraperitoneum, also referred to as the subperitoneal space, is located below the peritoneal reflection and surrounds the pelvic organs. The extraperitoneal spaces are susceptible to numerous diseases and pathologic processes, reflecting the diversity of structures they contain. These conditions can be congenital, inflammatory, infectious, neoplastic, or traumatic in nature. Imaging modalities such as CT and MRI are indispensable for identifying normal anatomy and characterizing pathologic processes. Patterns of disease spread in these compartments depend on the structures involved and the nature of the disease process. Malignancies often spread via lymphatic and hematogenous routes, while inflammatory and infectious processes tend to track along fascial planes. Vascular pathologic processes, including aneurysms and dissections, can cause secondary effects on adjacent structures. Understanding the anatomic landmarks, imaging features, and patterns of disease spread within the retroperitoneum and pelvic extraperitoneum is critical for accurate diagnosis and effective disease management. ©RSNA, 2026 Supplemental material is available for this article.