Introduction
Primary retroperitoneal masses may be categorized as solid or cystic and range from
benign to aggressive in behavior. Benign and malignant masses should be distinguished
whenever possible to avoid unnecessary surgical procedures.
Ultrasonography (USG) is the imaging modality performed initially due to its wide
availability, ease of performance, low cost, and absence of radiation exposure. It
plays an important role in characterization of mass lesions present in the retroperitoneum
based on its location, size, shape, extent, margins, solid or cystic nature, echotexture,
presence of calcification, and vascularity of the lesion on color Doppler. Presence
of any free fluid and associated abnormality in any solid organ or enlarged lymph
nodes can also be detected. However, USG examination of the retroperitoneum may be
technically limited because of excessive bowel gas or body habitus.
Multidetector computed tomography (CT) plays an important role in depicting the exact
compartmental localization of the masses into five retroperitoneal spaces, which include
the anterior pararenal space, posterior pararenal space, perirenal space, central
vascular space, and iliopsoas space.[1] It is currently the imaging modality of choice for the evaluation of morphology,
internal characterization, assessment of disease extent, and involvement of vessels
including the adjacent and distant structures.[2] It allows excellent reconstructions in planes other than the original scanning plane.
Isotropic multiplanar reconstructions of thin overlapping slices in the coronal and
sagittal plane provide excellent delineation of the location and extent of retroperitoneal
masses.
The appearance of retroperitoneal lesions on cross-sectional imaging may pose a diagnostic
challenge to the radiologist. Awareness of specific components of masses, tumor vascularity,
and specific patterns of spread helps in further narrowing the differential diagnosis.
Therefore, having knowledge of radiological features of various masses on cross-sectional
imaging offers valuable insights for differential diagnosis of retroperitoneal masses
and their management.
Normal Anatomy of Retroperitoneum
The retroperitoneum extends from the diaphragm to the pelvis. It extends between the
posterior parietal peritoneum anteriorly and the fascia transversalis posteriorly.[2] The retroperitoneum is divided into the anterior pararenal space, posterior pararenal
space, perirenal space, central vascular space, and iliopsoas space[1] ([Fig. 1]).
Fig. 1 Schematic diagram of retroperitoneal spaces. The anterior pararenal space lies between the anterior renal fascia and parietal peritoneum. It contains the pancreas,
duodenum, and the ascending–descending colon. The perirenal space lies between the anterior renal fascia and the posterior renal fascia and contains
kidneys, adrenal glands, ureters, blood vessels, and lymphatics. The posterior pararenal space lies between the fascia transversalis and the posterior renal fascia. The space only
contains fat. The central vascular space extends from D12 to L4–L5, located between the two perirenal spaces, behind the anterior
pararenal space, and in front of the spine. It contains the abdominal aorta and its
branches, the inferior vena cava and its afferent vasculature, lymphatic chains, and
the abdominal sympathetic trunk. The iliopsoas space lies posterior to the fascia transversalis and is generally considered to be retroperitoneal
even though it is behind the transversalis fascia because it is frequently involved
in processes that begin in the retroperitoneum.
The anterior pararenal space contains the pancreas, duodenum, and a part of the ascending
and descending colons. Kidneys, adrenal glands, blood vessels, lymphatics, and ureters
are in perirenal spaces. Fat is only present in the posterior pararenal spaces. The
central vascular compartment is located between the two perirenal spaces, behind the
anterior pararenal space and in front of the spine. It extends from D12 to the L4–L5
vertebra and contains the inferior vena cava (IVC) and its afferent vasculature, abdominal
aorta and its branches, lymphatic chains, and the abdominal sympathetic trunk. The
iliopsoas compartment contains the psoas major, iliacus, and psoas minor muscles.
Although the iliopsoas compartment is behind the transversalis fascia, it is still
considered retroperitoneal because it is frequently involved in processes that begin
in the retroperitoneum.[1]
The anterior and posterior pararenal spaces merge inferiorly, forming the infrarenal
retroperitoneal space, which communicates with the prevesical space and extraperitoneal
compartments of the pelvis. Because of loose connective tissue in the retroperitoneum,
tumors can have widespread extension before clinical presentation.[3]
Solid primary retroperitoneal masses are broadly classified according to the tissue
of origin. Four broad categories include mesodermal, neurogenic, germ cell, and lymphoid
masses ([Table 1]).[3]
Table 1
Classification of common solid primary retroperitoneal masses according to the tissue
of origin
|
Tissue of origin
|
Malignant masses
|
Benign masses
|
|
a) Mesodermal
|
|
Adipocytic
|
Liposarcoma
|
Lipoma
|
|
Smooth muscle
|
Leiomyosarcoma
|
Leiomyoma
|
|
Striated muscle
|
Rhabdomyosarcoma
|
Rhabdomyoma
|
|
Connective tissue
|
Malignant fibrous histiocytoma, chondrosarcoma, synovial cell sarcoma, and fibrosarcoma
|
Fibroma
|
|
b) Neurogenic
|
|
Nerve sheath
|
Malignant nerve sheath tumors
|
Schwannoma and neurofibroma
|
|
Chromaffin tissue
|
Malignant paraganglioma or pheochromocytoma
|
Paraganglioma and pheochromocytoma
|
|
Sympathetic nerves
|
Neuroblastoma
|
Ganglioneuroma and ganglioneuroblastoma
|
|
c) Germ cell neoplasms
Embryonic tissue
|
Teratoma (malignant) and primary extragonadal germ cell tumor (seminomatous and nonseminomatous)
|
Teratoma (mature and immature)
|
|
d) Lymphoid
|
–
|
Lymphoma
|
Cystic primary retroperitoneal masses are classified into neoplastic masses and non-neoplastic
masses ([Table 2]).[3]
Table 2
Classification of common cystic primary retroperitoneal masses
|
Type of mass and origin
|
Mass
|
|
a) Neoplastic
|
|
Mesothelial
Germ cell
Epithelial
|
Mesothelioma, cystic teratoma, mucinous cystadenoma or cystadenocarcinoma, serous
cystadenocarcinoma
|
|
Cystic change in solid neoplasm
|
Paraganglioma, neurilemoma, and sarcoma
|
|
b) Non-neoplastic
|
Hematoma, urinoma, and lymphocele
|
Malignant Retroperitoneal Tumors
In this section, we will review the imaging features of some common and uncommon malignant
retroperitoneal masses such as malignant lymphoma, liposarcoma, myxofibrosarcoma (MFS),
extra-adrenal neuroblastoma (NBL), and extra-skeletal Ewing's sarcoma.
Lymphoma
Lymphoma is the most common malignant retroperitoneal neoplasm, as well as being the
most common small round cell tumor,[4] and typically presents as infiltrative homogenous hypovascular masses around the
aorta or IVC, extending between and encasing structures without compressing them.[5] Upliftment of the great vessels results in “floating aorta” or “CT angiogram” sign
([Fig. 2]). Abdominal lymphomas are classified into solitary mass type, multiple nodular type,
and diffuse type.[6]
18F-fluorodeoxyglucose (18F-FDG) positron emission tomography (PET)/CT is the standard examination for the diagnosis,
staging, and in-treatment evaluation of lymphoma.[4] Heterogeneous or rim enhancement of lymph nodes due to necrosis with calcifications
is seen following radiochemotherapy.[7] The main differential diagnosis for lymphomatous lymph nodes is tuberculosis.[6] Mesenteric lymph nodes are more commonly involved in tuberculosis. It is often associated
with tuberculous peritonitis, which is characterized by omental caking, smudged mesentery,
and high-density ascites. Mesenteric lymph nodes are not commonly involved in Hodgkin's
disease. CT shows homogenous enhancement of the enlarged lymph nodes with fewer calcifications
in untreated lymphoma cases. Peritoneal and omental lymphomatosis is not often seen.[6]
Fig. 2 (A,B) Axial contrast-enhanced computed tomography (CT) of the abdomen in a 55-year-old
man shows a homogenously enhancing mass lesion in the central vascular space of the
retroperitoneum causing anterior and lateral displacement of the pancreas and bowel
loops, respectively. The lesion is closely abutting the liver, spleen, and bilateral
kidneys. It is encasing the aorta and displacing it anteriorly giving the “floating
aorta sign” (arrows in A), celiac trunk (notched arrow in A), right renal artery (thin arrow in B). On histopathology, it was proven to be a lymphoma of diffuse large B-cell type
(DLBCL).
Liposarcoma
Liposarcomas are mesenchymal tumors arising from adipose tissue. Retroperitoneal liposarcomas
are classified into five groups: well-differentiated liposarcoma, myxoid liposarcoma,
round cell liposarcoma, pleomorphic liposarcoma, and dedifferentiated liposarcoma.[8]
Well-differentiated liposarcomas appear as well-defined fat containing masses with
thin septa ([Fig. 3]). Calcifications or ossifications within a liposarcoma have proved to be a sign
of poor prognosis, often indicating dedifferentiation.[9] Well-differentiated liposarcomas almost always undergo dedifferentiation,[8] which is suggested by the additional presence of a focal, nodular nonlipomatous
region greater than 1 cm in size ([Fig. 4]).[10] Calcification is an important sign of dedifferentiation.[3] Myxoid liposarcomas appear as hypoechoic masses on USG. On CT and magnetic resonance
imaging (MRI), myxoid liposarcomas have a “pseudocystic” appearance[3] due to the extracellular myxoid matrix. They appear as low-attenuation masses in
comparison to muscle on CT with bright T2 signal on corresponding MRI. Slowly progressive
reticular type of contrast enhancement is seen following contrast administration[3] ([Fig. 5]), a feature that distinguishes these lesions from a cystic mass.[10] Round cell liposarcoma and pleomorphic liposarcoma exhibit soft-tissue tumor attenuation
and signal intensity with a minimal amount of fat.[8]
Fig. 3 Axial contrast-enhanced computed tomography (CT) of the abdomen shows a large retroperitoneal
mass of fat attenuation (arrow) displacing the right kidney. On histopathology, it proved to be a well-differentiated
liposarcoma.
Fig. 4 Axial contrast-enhanced computed tomography (CT) of the abdomen shows a well-defined
solid enhancing mass with few areas of fat density (arrow) and multiple enhancing vessels (thin arrow) in the left anterior pararenal space of the retroperitoneum. It was diagnosed as
dedifferentiated liposarcoma on histopathology.
Fig. 5 In another patient, axial contrast-enhanced computed tomography (CT) shows an ill-defined
diffusely infiltrative multicompartmental lesion of fat attenuation in the left lumbar
and iliac region showing internal fat stranding, eccentric coarse calcifications,
and enhancing nodular septations (arrow). It was proven to be myxoid liposarcoma on histopathology.
Myxofibrosarcoma
MFS are fibroblastic lesions that show a spectrum of cellularity, mitotic activity,
and nuclear pleomorphism ranging from hypocellular lesions with minimal cytological
atypia to more cellular lesions.[11] Low-grade MFS invariably show more than 75% myxoid change.[12] They have the unusual characteristic of extending along the fascial planes well
beyond the primary center of the lesion.[13] Tail sign, which is defined by the tendency of the tumor to spread along the fascial
planes in a curvilinear fashion, seen on MRI is a moderately sensitive and specific
sign for MFS.[12] Tail sign is not only valuable for suggesting the diagnosis of MFS but its recognition
is also essential in preoperative planning for complete tumor resection ([Fig. 6]).
Fig. 6 (A,B) Axial contrast enhanced computed tomography (CT) of the abdomen in a 50-year-old
male shows a large heterogeneously enhancing abdominopelvic mass lesion with thin
septa, calcification (arrow in B). The mass is abutting the left iliopsoas muscle with loss of intervening fat planes
posteriorly. On histopathology, it was proven to be a low-grade myxofibrosarcoma.
The red square symbol in figure 6A shows the attenuation of the mass, the parameters
of which are given in the box below.
Extra-Adrenal Neuroblastoma
NBL, ganglioneuroblastoma, and ganglioneuroma are tumors of ganglion cell origin derived
from the primordial neural crest cells.[14] The most common primary site for NBL is adrenal medulla (35%) and the extra-adrenal
paraspinal ganglia (30–35%).[15] NBLs are heterogenous poorly circumscribed masses. Eighty percent to 90% show coarse,
amorphous, and mottled calcifications ([Fig. 7]).[16] They can demonstrate extension across the midline and into adjacent body cavities.
Vascular invasion is not a feature of NBLs.[17]
Fig. 7 (A,B) Axial and coronal contrast-enhanced computed tomography (CT) of the abdomen in a
4-year-old girl shows an ill-defined heterogeneously enhancing soft-tissue mass with
internal foci of coarse calcifications (arrow in A) centered in the right anterior pararenal space of the retroperitoneum displacing
the inferior vena cava (IVC), left renal vein, and pancreas anteriorly. No foci of
fat attenuation are noted. No spinal canal extension was noted. The right adrenal
gland is normal (thin line arrow in B). It was a proven case of extra-adrenal neuroblastoma.
Extraskeletal Ewing's Sarcoma
Extraskeletal Ewing's sarcoma (EES) can occur anywhere in the body and can be classified
into central/trunk and peripheral/extremity. On imaging, EESs show extensive necrosis
and hemorrhage. Calcification is uncommon in EES. Tumors in the abdomen tend to have
infiltrative margins with invasion of the adjacent structures and displace adjacent
structures rather than encase them ([Fig. 8]). Retroperitoneal EESs are difficult to differentiate from other retroperitoneal
tumors in adults such as the renal cell carcinoma and adrenocortical carcinoma. Features
in favor of EESs are their earlier age of presentation, absence of metastatic lymphadenopathy,
and calcification. Abdominal EESs are mostly unilateral and do not cross the midline.[18]
Fig. 8 (A,B) Axial contrast-enhanced computed tomography (CT) of the abdomen in an 11-year-old
girl shows a large right-sided abdominopelvic heterogeneously enhancing predominantly
cystic retroperitoneal mass lesion (arrow in A) with extensions into the neural foramina and spinal canal, bony destruction, and
lytic vertebral lesions (thin arrow in B). On histopathology, it was proven to be extraskeletal Ewing's sarcoma.
Benign Retroperitoneal Tumors
In this section, we will review the imaging features of some benign retroperitoneal
masses like teratoma, paraganglioma, schwannoma, and urinoma.
Teratoma
Teratomas may be composed of mature or immature tissues deriving from the three pluripotent
germ cell layers.[19] Mature cystic teratomas (MCTs) are characterized as cystic tumors with fat attenuation
and Rokitansky's protuberance.[20] Rokitansky's nodules are soft-tissue protuberance within the cyst cavity. Bone or
teeth, if present, tend to be located with Rokitansky's nodule ([Fig. 9]).[21] Immature teratomas (ITs) contain variable quantities of immature neural tissue.
On cross-sectional imaging, ITs appear as large solid masses containing coarse calcifications
and small foci of fat with concomitant hemorrhage.[22] Chemotherapy treatment of ITs can result in tissue maturation. This phenomenon is
called “retroconversion”; the ITs look more like MCTs.[23] CT findings suggesting maturation are increasing density of tumor masses, appearance
of some cystic components, fatty areas, and curvilinear and punctate calcifications.
These masses do not invade the surrounding soft tissues or bone but may gradually
compress them.[24]
Fig. 9 (A,B) Axial and coronal contrast-enhanced computed tomography (CT) of the abdomen reveals
a large heterogenous predominantly cystic retroperitoneal mass lesion causing anterior
displacement of the stomach and pancreas (arrow in A) with areas of fat (notched arrow in B) and calcification (mean ∼314 HU; thin arrow in B) in the left hypochondrium and lumbar regions. Coronal reformatted CT image (B) showing superolateral displacement of the spleen, mass effect, and inferior displacement
of the left kidney. On histopathology, it was confirmed to be a mature cystic teratoma.
Paraganglioma
Paragangliomas are rare tumors of chromaffin cell origin, arising from the extra-adrenal
paraganglion cells of sympathetic or parasympathetic nervous systems. The most common
site of paragangliomas in the abdomen is the organ of Zuckerkandl located in the para-aortic
region near the origin of the inferior mesenteric artery ([Fig. 10]).[25] They appear as heterogeneous masses with necrosis, hemorrhage, and/or calcifications.
Avid contrast material enhancement is often noted because of their hypervascular nature,
especially peripheral.[26] On T2-weighted MRI, paragangliomas show diffuse high signal intensity known as “lightbulb”
sign.
Fig. 10 Axial (A) noncontrast and (B) contrast-enhanced computed tomography (CT) of the abdomen in a 50-year-old woman
reveals a well-defined multiloculated solid cystic lesion located in the left paraaortic
location at the level of the origin of the inferior mesenteric artery showing fluid–fluid
level (thin arrow in A) and a hyperenhancing solid component (arrow in B). Anterosuperiorly, the mass is seen to abut the fourth part of the duodenum and
duodenojejunal junction displacing it anteriorly (vertical arrow in B). On histopathology, it was proven to be a paraganglioma.
Schwannoma
Schwannomas are benign nerve sheath tumors of Schwann cell origin. On cross-sectional
imaging, schwannomas are well-circumscribed masses, most commonly located in the paravertebral
or presacral location. Target sign is seen on MRI, consisting of a central low to
intermediate T2-signal intensity due to fibrous tissue surrounded by a peripheral
high signal intensity of myxoid tissue.[27] It may be seen in both schwannomas and neurofibromas. Larger schwannomas are more
likely to undergo degenerative changes, which include cyst formation, calcification,
hemorrhage, and hyalinization ([Fig. 11]).[25] Ancient schwannoma refers to a long-standing lesion with advanced degenerative changes.[3]
Fig. 11 (A,B) Axial contrast-enhanced computed tomography (CECT) of the abdomen in a 17-year-old
adolescent girl shows a large well-defined nonenhancing (mean ∼38 HU on CECT and 32
on noncontrast-enhanced computed tomography [NCCT], plain scan not shown) lesion in
the right suprarenal region. No peripheral rim enhancement or enhancing soft-tissue
component is seen. The lesion does not show any focus of fat/calcification within.
The right adrenal gland is seen separately from the lesion (arrow in A). On histopathology, it was proven to be a schwannoma.
Urinoma
Urinoma is a collection of extravasated urine that is found secondary to trauma or
iatrogenic causes. They appear as well-defined lesions of fluid attenuation in the
retroperitoneum, more commonly in the perirenal space. On CT, they are seen as well-circumscribed
fluid collections showing increasing attenuation caused by contrast-enhanced urine
entering the urinoma on excretory phases ([Fig. 12]).[25]
Fig. 12 (A) Axial contrast-enhanced computed tomography (CT) of the abdomen in 15-minute delayed
phase in a 51-year-old woman with a history of abdominal surgery shows a large well-defined
collection in the perinephric space. There is contrast spurt from the right ureter
(thin arrow in A) and subsequent contrast layering in collection (arrow in A) resulting in the formation of urinoma secondary to ureteric injury. (B) Sagittal reformatted CT image shows cranial displacement of the right kidney.
