The gamut of primary retroperitoneal masses: multimodality evaluation with pathologic correlation.

The retroperitoneum is a large space where primary and metastatic tumors grow silently before clinical signs appear. Neoplastic retroperitoneal diseases may be solid or cystic, primary or secondary and range from benign to aggressive in behavior. Retroperitoneal neoplasms are notable for their widely disparate histologies. The solid primary retroperitoneal neoplasms are extremely uncommon and can be classified based on their tissue of origin into three main categories: mesodermal tumors, neurogenic tumors, and extragonadal germ cell tumors. These tumors can grow to a large size before clinical symptoms occur or become palpable. When symptoms do occur, they are nonspecific. The majority of these masses are malignant and imaging plays a pivotal role in the detection, staging, and pre-operative planning. Benign and malignant masses should be distinguished whenever possible to avoid unnecessary surgical procedures. Macroscopic fat, calcification, necrosis, vascularity, and neural foraminal widening are common imaging features helping for tumor differentiation. Meticulous cross-sectional imaging can triage the patient to the most appropriate therapy. Tumor morphology dictates imaging character, and biologic activity is reflected by positron emission tomography (PET). Complete surgical excision with tumor free margins is essential for long-term survival. Biopsy should be performed in consultation with surgical oncology to avoid complicating curative surgery. This pictorial essay illustrates the spectrum of multidetector computed tomography (MDCT) imaging findings in common and uncommon primary retroperitoneal masses, with an emphasis on cross-sectional imaging features for an adequate tumor characterization and staging.