Approach to Benign

Retroperitoneal Masses

Primary retroperitoneal neoplasms

Of the Primary retroperitoneal neoplasms (Retroper. Masses not arising from major retroperi. organs are - outside the major organs), 70%–80% are malignant in nature, and these account for 0.1%–0.2% of all malignancies in the body.

Primary retroperitoneal neoplasms are diverse group of benign and malignant tumors that arise within the retroperitoneal space but outside the major organs in this space.

PRIMARY RETROPERITONEAL NEOPLASMS (Retroper. Masses not arising from major retroperi. organs are - outside the major organs) :

Learning objectives Determine whether a tumor is located in the

retroperitoneal space.

Identify the organ of origin of a retroperitoneal tumor.

Describe specific imaging findings in a variety of retroperitoneal tumors.

Tumor Location

Anterior displacement of retroperitoneal organs (eg, kidneys, adrenal glands, ureters, ascending and descending colon,pancreas, portions of the duodenum and major vessels) strongly suggests that the tumor arises in the retroperitoneum

Identification of the Organ of Origin

•Beak sign•Embedded organ sign•Phantom organ sign•Prominent feeding artery sign

Beak Sign.—When a mass deforms the edge of an adjacent organ into a “beak” shape, it is likely that the mass arises from that organ (beak sign).

On the other hand, an adjacent organ with dull edges suggests that the tumor compresses the organ but does not arise from it

Positive beak sign Negative Beak Sign

BEAK SIGN

Phantom (Invisible) Organ Sign.—When alarge mass arises from a small organ, the organsometimes becomes undetectable. This is knownas the phantom organ sign.

Embedded Organ Sign

• The embedded organ sign occurs when the organ of origin appears embedded in a mass rather than deformed by it.

• When a tumor compresses an adjacent plastic organ (eg, gastrointestinal tract, inferior vena cava) that is not the organ of origin, the organ is deformed into a crescent shape.

Prominent Feeding Artery Sign Hypervascular masses are often supplied by feeding arteries that are prominent enough to be visualized at CT or MR imaging, a finding that provides an important key to understanding the origin of the mass

SPECIFIC FEATURESSpecific Patterns of Spread:

• Lesions That Extend Between Normal Structures.

Lymphangiomas , lymphomas & ganglioneuromas

Lesions That Extend along Normal Structures. Tumors of the sympathetic ganglia (ie, paragangliomas, ganglioneuromas) tend to extend along the sympathetic chain and have an elongated shape.

LYMPHANGIOMAS

Transverse contrast-enhanced abdominal(a)and pelvic(b)CT scans show a multiloculated, low-attenuation cystic mass that extends between normal anatomic structures in the peritoneal cavity and retroperitoneum

"CT angiogram sign" or "floating aorta sign"

Characteristic tumor components• Fat• Myxoid stroma• Necrosis• Cystic portion• Vascularity• Small round cells

Fat• A mass that is homogeneous and well defined and consists almost entirely

of fat represents lipoma

• The presence of fat is easily recognized owing to its low attenuation at CT or its high signal intensity at T1-weighted MR imaging with loss of signal intensity on fat-suppressed images. The presence (or absence) of fat limits the differential diagnosis.

• Teratomas are also characterized by the presence of fat and mature teratomas can be characterized by the presence of fluid attenuation or signal intensity, fat-fluid levels, and calcifications

• Characteristic tumor components [Lipoma]

FAT

Abdominal radiograph shows a huge radiolucent mass.(b)TransverseCT scan of the abdomen shows that the mass is composed primarily of fat.(c)On a T1-weighted MR image, the mass has homogeneous high signal intensity and compresses the kidney upward

Axial CT image of a 21-year-old woman with a mature teratoma shows a well-defined heterogeneous mass that demonstrates fat (arrow) and teeth (arrowhead).

Mature teratoma.

Myxoid Stroma• Myxoid stroma is characterized pathologically by a mucoid matrix that

is rich in acid mucopolysaccharides. • Myxoid stroma appears hyperintense on T2-weighted MR images and

shows delayed enhancement after injection of contrast medium.• Tumors that commonly contain myxoid stroma include • neurogenic tumors (schwannomas, neurofibromas, ganglioneuromas,

ganglioneuroblastomas, malignant peripheral nerve sheath tumors), • myxoid liposarcomas, and• myxoid malignant fibrous histiocytoma.

• Schwannomas are the most common tumor of peripheral nerves. • Schwannomas are well encapsulated and contain cells that are identical to Schwann cells. • Their MR imaging appearance depends on the types of tissue they contain. Myxoid tissue

is hyperintense on T2-weighted images, cellular tissue is hypointense on both T1- and T2-weighted images, and solid fibrous tissue enhances on contrast-enhanced images. • Neurofibromas tend to have high signal intensity on T2-weighted MR images and are

often multiple and associated with neurofibromatosis. • Ganglioneuromas are typically located along the sympathetic chain and tend to be larger,

more rounded, and contain calcification more frequently than nerve sheath tumors. The relatively younger age of affected patients may aid in differentiating ganglioneuromas from other neurogenic tumors.

Schwannoma

Transverse contrast-enhanced CT scan shows a well-marginated low-attenuation mass interposed between the portal vein, superior mesenteric artery, aorta, and inferior venacava.(b)On a T2-weighted MR image, the mass appears mostly hyperintense.

NEUROFIBROMA GANGLIONEUROMA

Transverse contrast-enhanced CT scan of the abdomen shows a well-marginated, minimally enhancing mass in the right paraaortic region.(b)On aT2-weighted MR image, the mass appears hyperintense

Neurofibromas in a patient with neurofibromatosis type 1. Sagittal T2-weighted MRimage shows multiplehyperintense masses in thepresacral region

Necrosis• Necrotic portions within tumors have low attenuation without

contrast enhancement at CT and are hyperintense at T2-weighted MR imaging. • Necrosis is usually seen in tumors of high-grade malignancy such as

leiomyosarcomas.• When they occur in the retroperitoneum, leiomyosarcomas tend to

develop massive cystic degeneration. They have central necrosis more commonly than other sarcomas, whereas fat and calcifications are not typically present

NECROSIS

LEIOMYOSARCOMA

Transverse contrast-enhanced CT scan reveals a huge mass adjacent to the left kidney that displaces the spleen and pancreas anteriorly. The mass has heterogeneous enhancement with central nonenhancing foci that suggest necrosis. Enhanced vessels (arrow) are seen to penetrate the mass, afinding that reflects hypervascularity.(b)On a T2-weighted MR image, the mass is heterogeneous but relatively hypointense. Thecentral portion has high signal intensity (arrow), afinding that represents necrosis

Paraganglioma

Transverse unenhanced CTscan shows a large paraaortic mass with a fluid-fluid level.

Cystic Portion• Some tumors are completely cystic in appearance. These include

lymphangiomas and mucinous cystic tumors. • Solid tumors with a partially cystic portion include neurogenic tumors

Small Round Cells• At T2-weighted MR imaging, tumors composed of small round cells

often appear as homogeneous masses with relatively hypointense areas representing densely packed cellular components.• Lymphomas are the most commonly encountered tumors composed

of small round cells. They are homogeneous, with minimal contrast enhancement at CT and relatively low signal intensity at T2-weighted MR imaging. An exception is primitive neuroectodermal tumor (PNET), which often appears heterogeneous at MR imaging

SMALL ROUND CELLSLYMPHOMA PNET

T2-weighted MR image shows a presacral mass with homogeneous low signal intensity that represents densely packed small round cells

Transversecontrast-enhanced CT scan shows a large, partially illdefined mass with heterogeneous enhancement. Thespleen and the left kidney are displaced anteriorly.(b)On a T2-weighted MR image, the mass appears heterogeneous with interpersed high-signal-intensity spots.

Vascularity• Extremely hypervascular tumors: Paragangliomas and

Hemangiopericytomas.

• Moderately hypervascular tumors: Myxoid liposarcoma, malignant fibrous histiocytomas, leiomyosarcomas, and many other sarcomas.

• Hypovascular tumors: Low-grade liposarcomas, lymphomas, and many other benign tumors.

PARAGANGLIOMA

Solid Non-neoplastic Masses

1. Pseudotumoral lipomatosis,

2. Retroperitoneal fibrosis,

3. Erdheim-Chester disease,

4. Extramedullary hematopoiesis

Lipomatosis

Lipomatosis is a benign metaplastic overgrowth of mature unencapsulated white fat.

Lipomatosis is seen commonly in the pelvis and along the perirectal and perivesicular spaces and is seen less commonly in the abdominal retroperitoneum.

CT and MR imaging show excess fat in the pelvis crowding the anatomic structures, with a few fibrous tissue strands but no soft-tissue mass or enhancement.

A pear-shaped bladder is produced by symmetric compression and displacement of the bladder caused by pelvic lipomatosis.

The lower portions of the ureters may be pinched medially, with resultant hydroureteronephrosis.

Retroperitoneal Fibrosis

Retroperitoneal fibrosis is an uncommon collagen vascular disease of unknown cause that can mimic a retroperitoneal tumor.

Retroperitoneal fibrosis is typically idiopathic (>70% of cases) and is likely autoimmune in origin.

It can be secondary to drugs, malignancy, hemorrhage, inflammatory conditions, infection, radiation, chemotherapy, renal trauma, and amyloidosis.

More common in males (3:1),

40–60-year age group

CT - irregular plaque like soft-tissue mass in the retroperitoneum, located around the aortic bifurcation and extending along the iliac arteries and involving the ureters, duodenum, pancreas, and spleen.

It does not displace the aorta and IVC anteriorly, as lymphoma or metastatic nodes often do, but causes tethering of these structures to the underlying vertebrae.

Avid enhancement is seen in the active stages of retroperitoneal fibrosis with little or no enhancement in the chronic phase.

MR-

high signal intensity on T2 WI in the acute phase of the disease, with early contrast enhancement, and shows low signal intensity in the chronic fibrosing phase, with delayed enhancement.

Erdheim-Chester Disease

It is a rare non-Langerhans form of histiocytosis of unknown origin.

Extraskeletal lesions are seen in 50% of cases, with retroperitoneal involvement seen in one-third.

Radiography shows characteristic bilateral symmetric osteosclerosis of the metaphyseal-diaphyseal region of the long bones, with sparing of the epiphysis and lesser involvement of the flat bones and axial skeleton.

Retroperitoneal involvement with Erdheim-Chester disease characteristically produces a soft-tissue rind of fibrous perinephritis surrounding the kidneys and ureters, which can result in renal failure.

Extramedullary Hematopoiesis

It is seen in hemoglobinopathies, myelofibrosis, leukemia, lymphoma, and carcinomas.

The typical CT appearance is hyper- or isoattenuating round or lobulated masses in the paravertebral region, with or without macroscopic fat.

The MR imaging appearance is variable.

Low signal intensity can be seen on T1- and T2-weighted

images because of the red marrow or hemosiderin content.

High signal intensity may be found on T1- and T2-weighted images

because of fatty tissue.

Enhancement is variable and often mild and there is no associated

bone destruction or calcification.

Lymphangioleiomyomatosis• LAM is a rare idiopathic disorder that occurs in women of childbearing

age and predominantly affects the lungs, less often the mediastinum and retroperitoneum. The disease is characterized by a hamartomatous proliferation of smooth muscle cells that involves lymph nodes and lymphatics, resulting in dilation of lymphatic spaces with cyst formation.• On CT or MRI, cystic lung disease, chylous pleural effusions,

pneumothoraces, and renal angiomyolipomas are most frequently encountered. Retroperitoneal or pelvic lymphangioleiomyomas are less common, occurring in up to 20% of patients. These appear as low-attenuation high-T2-SI cystic masses with enhancing walls, sometimes with delayed enhancement of the internal contents

CT and MR imaging show a well-defined unilocular homogeneous cystic mass.

large thin-walled unilocular or multilocular cystic mass with attenuation values ranging from that of fat (caused by chyle) to that of fluid.

sagittal T2-weighted (B) images through pelvis demonstrate well-circumscribed multiloculated lesion (arrows) within presacral space with variable T1 and T2 SI within locules due to variable amounts of proteinaceous or hemorrhagic fluid conten

axial T2-weighted (B) images through pelvis demonstrate small unilocular lesion (arrows) medial to left externaliliac vessels, with fluid attenuation and very high T2 SI relative to skeletal muscle

Axial contrast-enhanced CT MIP image during excretory phase of enhancement demonstrates leakage of contrast material (arrows) from medial left renal collecting system into perirenal space in keeping with perirenal urinoma.

Diagnostic Clues for a Retroperitoneal Mass

STEP-1 :Identify the lesion whether it is Intra-abdominal or Retroperitoneal.

STEP-2:Identifying Primary Retroperitoneal lesions

STEP-3:Identifying retroperitoneal lesions arising from retroperitoneal structures

Flow chart-Approach for diagnosis of retroperitoneal lesions.

References :Imaging of Uncommon Retroperitoneal Masses.

Prabhakar Rajiah et al.Primary Retroperitoneal Neoplasms: CT and MR

Imaging Findings with Anatomic and Pathologic Diagnostic Clues. Mizuki Nishino et al.

CT & MRI OF THE WHOLE BODY BY JOHN.R.HAAGA

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