Anatomy of the Retroperitoneal Space

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 (Gerota’s fascia), posterior renal fascia (Zuckerkandl’s fascia), and lateroconal fascia (Fig. 1).

Fig 1: Anatomy of retroperitoneal space. APS- Anterior Pararenal Space; PRS-...

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:

• Anterior: anterior renal fascia (Gerota)
• Posterior: posterior renal fascia (Zuckerkandl)
• 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 (Zuckerkandl)
• 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.

• Solid retroperitoneal neoplastic lesions have been classified according to the tissue of origin and broadly include mesenchymal tumors/sarcomas, lymphoid tumors, neurogenic and germ cell tumors (Fig. 2).
  

  Fig 2: Table 1. Classification of retroperitoneal neoplastic lesions according to the...

• We propose a multistep system that will help the radiologist to narrow the differential diagnosis:

1. First step: Confine the lesion to the retroperitoneal space

The mass effect - anterior displacement of the normal retroperitoneal organs as well as retroperitoneal major vasculature and their branches, strongly suggests that the lesion arises from or is localized to the retroperitoneum.

2. Second step: Classify it as primary retroperitoneal

Labeling a lesion as a primary retroperitoneal mass requires the exclusion of the tumor origin from retroperitoneal organs. The following  radiological signs aid in determining these characteristics:

• Beak or claw sign
• Embedded organ sign
• Phantom (invisible) organ sign
• Prominent feeding artery sign.

• Beak or claw sign

If a mass creates an obtuse angle and dull edges as it compresses an organ, it points that is primary and does not originate from that organ (negative beak sign) – (Fig. 3)

Fig 3: (A) Axial contrast-enhanced CT image of a 40-year-old man reveals a...

• Embedded organ sign

If a tumor deforms a plastic organ (e.g. gastrointestinal tract, IVC) and makes it assume a crescentic shape, the tumor does not originate from it (negative embedded sign) – (Fig.4).

Fig 4: Negative “embedded organ” sign (A) Post-contrast axial CT image shows a...

• Phantom (invisible) organ sign

When a large mass arises from a small organ sometimes that organ becomes undetectable (Fig.5).

Fig 5: “Phantom organ” sign. (A) Contrast-enhanced axial CT image of a 40-year-old...

• Prominent Feeding artery sign

Hypervascular masses are often supplied by feeding arteries that are prominent enough to be visualized at CT or MRI, which can help in identifying the organ of origin (Fig.6)

Fig 6: Prominent feeding artery. MIP reconstruction of coronal and axial...

3. Third step: to assess the structure of the lesion- exclude cystic lesions

Cystic lesions must be excluded using imaging findings as:

• CT: a simple cyst is a thin-walled, homogeneous lesion with near water attenuation (0-20 UH). The content of a cystic lesion is variable and can be divided into 3 categories:
• Homogenous near-water attenuation: simple fluid;
• Lower-than-water attenuation: chylous fluid and lipid contents;
• Higher-than-water attenuation: homogenous or heterogenous owing to the presence of proteinaceous material, hemorrhage, or necrotic tissue.
• MRI: simple cystic lesions have marked T2-weighted hyperintensity and low T1-weighted signal intensity.

            The presence of proteinaceous or hemorrhagic products in the cyst decreases T2w hyperintensity and increases T1w signal.

• Cystic tumors- the presence of a thick irregular wall or solid nodular components in a multilocular cyst should raise concern for malignant neoplasm [4] (Fig.20-21).

It is important to analyze all the components of a solid tumor to narrow the differential diagnosis (fig. 7):

Fig 7: Differential diagnosis of solid retroperitoneal lesions according to the...

• Soft tissue : attenuation >+20UH(fig.13);

• Fatty component:

CT: attenuation < - 20 UH;

MRI: hyperintense on T1w, moderately hyperintense in T2w, and hypointense on fat-suppressed images – (fig.11-12).

Fig 11: Well-differentiated liposarcoma. (A) coronal arterial phase CECT; (B) axial...

Fig 12: De-differentiated liposarcoma: (A) arterial phase axial CECT; (B) venous phase...

• Calcifications:

- are best seen on CT;

- on MRI are seen as markedly hypointense areas when they are large;

- when present it can have specific patterns as seen in undifferentiated pleomorphic sarcoma: a ring-like pattern near the periphery or a speckled pattern centrally.

• Hemorrhage (imaging characteristics depending on the age of the hemorrhage)- (fig.19);

• Myxoid Stroma:

- CT: hypodense to muscle;

- MRI high fluid-like signal on T2, hypointense on T1;

- after contrast medium administration: delayed heterogeneous enhancement pattern, due to the slow, progressive accumulation of contrast within the extracellular myxoid matrix.

• Necrotic component (fig. 19):

- CT: hypoattenuating ;

- MRI: T1-weighted images: low-intensity signal; T2-weighted images: hyperintense;

- without contrast enhancement.

Fig 19: Contrast-enhanced CT images of a 56-year-old man (A) arterial phase (B) venous...

4. Fourth step: Vascular and enhancement pattern of the lesion (fig.8):

There have been described 3 enhancement patterns 

1. Early enhancement with rapid wash-out- benign lesions
2. Early enhancement with delayed/imperceptible wash-out – mostly malignant
3. Delayed enhancement – benign and malignant masses with myxoid or fibrotic components [5].

Fig 8: Differential diagnosis of solid retroperitoneal lesions based on the...

Fig 16: Abdominal axial MRI images illustrating a left paraaortic nodular lesion (A)...

4. Fifth step: Pattern of spread and growth(fig. 9)

Another fundamental characteristic to use when assessing a retroperitoneal tumor is the growth rate in time :

• slow growth rather suggests benign etiology;
• fast-growing mass is more worrisome for malignancy.

 The mantle growth pattern refers to lesions involving adjacent structures with no obvious signs of infiltration.

Fig 9: Scheme for the differential diagnosis of solid retroperitoneal lesions...

5. Sixth step: Particular signs that can suggest a specific diagnosis (fig.10)

• Target Sign: central area of low signal (fibrous tissue) surrounded by a rim of intense signal (myxoid stroma) in T2WI.

• Bowl of fruit sign: a mosaic of mixed low, intermediate, and high signal intensities on T2WI as a result of a combination of solid components, cystic degeneration, hemorrhage, myxoid stroma, and fibrous tissue.

• Whorled appearance: a linear or curvilinear structure appearing hypointense in T2, corresponding to a band of Schwann cells and collagen fibers [5].

• Flow voids: are a form of MRI artifact and refer to a signal loss occurring in moving fluids (e.g. blood) and are generally synonymous with vascular patency.

• Speckled enhancement: seen in T1WI after contrast media administration and corresponds to intratumoral structures similar to septa.

• CT angiogram sign: the tumor is infiltrative and spreads between normal retroperitoneal structures surrounding and engulfing vessels (aorta and IVC) without compressing their lumen (fig. 14).

• The “Floating aorta” sign: when a paraspinal mass elevates and displaces the aorta from the vertebral column and does not cause stenosis or invasion (fig.14).
  

  Fig 14: Retroperitoneal non-Hodking lymphoma: Arterial phase CECT (A) coronal plane...

Fig 7: Differential diagnosis of solid retroperitoneal lesions according to the...

6. Seventh step: Clinical Context

• Due to the high heterogeneity, overlapping, and nonspecific imaging findings of retroperitoneal tumors, knowledge of the clinical context is crucial for the definite diagnosis.
• Some tumors may be functional like paraganglioma and may be detected earlier, owing to symptoms of catecholamine excess such as hypertension, tachycardia, and diaphoresis (fig. 19).