Introduction

Primary retroperitoneal masses constitute a rare group of lesions which do not originate from a specific retroperitoneal organ. Majority of these mass lesions are malignant (~ 70-80%) and are associated with high mortality rates owing to non specific clinical features and late presentation. Determining the anatomic extent and key imaging features of primary retroperitoneal masses is crucial for providing an accurate diagnosis and in selecting the appropriate clinical management. By early detection and proper evaluation of retroperitoneal masses through cross sectional imaging modalities, the radiologist can aid in early treatment and thus ensure better prognosis.

Anatomy of the retroperitoneal spaces

The 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, posterior renal fascia, and lateroconal fascia

Fig 1: Anatomy of retroperitoneal space. LK=left kidney, RK=right kidney ; References .Chong, S., Shin, H. S., & Lee, Y. C. (2008). Retroperitoneal fascial anatomy: Current concepts and implications in disease processes. Radiographics, 28(6), e36

Compartments

• Anterior Pararenal Space

Limits:

• Anterior: parietal peritoneum
• Posterior: anterior renal fascia
• Laterally: lateroconal fascia.

Content: ascending and descending colon, duodenum, pancreas.

• Perirenal Space

Limits:

• Anterior: anterior renal fascia
• Posterior: posterior renal fascia
• The renal fascia joins and closes the perirenal space resembling an inverted cone with its tip in the iliac fossa.

Content: kidneys, adrenal glands, ureters.

• Posterior pararenal space

Limits:

• Anterior: posterior renal fascia
• Posterior: fascia transversalis.

Content: fat.

• The great vessel compartment

Limits:

• between perirenal spaces;
• Anterior: the anterior perirenal space;
• Posterior: the spine, extending from T12 to L5.

Contents:  abdominal aorta and its branches, inferior vena cava and its tributaries; lymphatic chains; abdominal sympathetic trunk.

Before assessing the specific imaging characteristics of retroperitoneal masses, it is important to first establish whether the mass originates from the retroperitoneal soft tissue or a retroperitoneal organ. This distinction is clinically significant, as the composition of the lesion often helps in assessing whether it is likely to be malignant or benign (2).

We propose a structured approach to help radiologists refine the differential diagnosis:

Determine if the lesion is confined to the retroperitoneal space1. Tumor location : displacement of normal retroperitoneal organs, major vasculature, and their branches anteriorly strongly indicates that the lesion is situated within the retroperitoneum (4).

2. Determine organ of origin:

• Beak or claw sign
  

  Fig 2: (A) Axial contrast-enhanced CT image reveals a mass that demonstrates a negative beak sign in relation to the right kidney. (B) Schematic diagram illustrating the negative beak sign. References : Iafrate, F., Ciolina, M., Venanzi, R., et al. (2016). Retroperitoneal tumors: A review of imaging features and differential diagnosis. Acta Radiologica, 57(5), 481–489
    - When a mass forms an acute angle or a "beak"-like shape at the edge of an adjacent organ, it suggests an origin from that organ. In contrast, if the mass creates obtuse angles and smooth edges while compressing an organ, it is likely external to it (2).  
• Embedded organ sign- A mass lesion that does not originate from a hollow or malleable organ will deform it into a crescent shape, indicating a negative embedded organ sign. In contrast, masses arising from an organ tend to encase it, making the organ appear “embedded”
• Phatom organ sign - Large masses arising from small organs can obscure the native organ on imaging, rendering it “invisible.” (10)
  

  Fig 3: Axial contrast-enhanced CT image reveals absence of phantom sign which suggests primary retroperitoneal pathology. References :Kawashima, A., Sandler, C. M., Ernst, R. D., et al. (1999). Imaging of non-vascular retroperitoneal pathology. Radiographics, 19(1), 151–181.
   
• Prominent feeding artery- Hypervascular masses may have a prominent feeding vessel visible on CT, aiding in determining their origin.

Approaches for refining the differential diagnosis of primary retroperitoneal lesions

The following is a 3-step algorithmic approach to confirming the retroperitoneal origin of a mass and establishing an accurate diagnosis by systematically excluding differential possibilities .

First step : determine the lesion is retroperitoneal (displacing retroperitoneal organs) and to exclude organ of origin

Fig 14: A scheme for the approach to abdominal or pelvic mass lesion.

Second step:  Based on the imaging appearance, classify the lesion broadly as solid or cystic

Fig 15: Cystic retroperitoneal lesions

• it important to analyze all the components of a solid tumor to narrow the differential diagnosis
  

  Fig 16: A scheme for the differential diagnosis of solid retroperitoneal lesions according to composition

 Third step: Vascular and enhancement pattern of the lesion

Fig 17: A scheme for the differential diagnosis of solid retroperitoneal lesions according to vascularity

• There have been described 3 enhancement patterns 

1. Early enhancement with rapid wash-out- benign lesions
2. Early enhancement with delayed wash-out – mostly malignant
3. Delayed enhancement – myxoid or fibrotic components 

Forth step: Location and pattern of spread and growth

Fig 18: Location and pattern of spread and growth

Solid primary retroperitoneal masses

Solid primary retroperitoneal masses can be broadly divided into neoplastic and non neoplastic categories

Fig 4: Solid primary retroperitoneal masses. References: Ganeshan, D., Morani, A., Elsayes, K. M., et al. (2016). Imaging features of primary retroperitoneal neoplasms: Beyond the usual suspects. Abdominal Radiology, 41(1), 219–232.