The retroperitoneum is the compartmentalized space located behind the posterior abdominal wall. It is divided into three compartments - the anterior pararenal space, perirenal space, and posterior pararenal space. The perirenal space contains the kidney and related structures. A variety of pathologies can occur in the retroperitoneum including sarcomas like liposarcoma, neurogenic tumors, and benign lesions such as schwannomas or lipomas. Imaging plays an important role in characterizing these retroperitoneal masses.
3. BOUNDARIES OF RETROPERITINEUM
Abdominal retroperitoneum is bounded
• anteriorly by the posterior parietal peritoneum,
• posteriorly by the transversalis fascia
• extends craniocaudally from the diaphragm to the
pelvic brim.
4. COMPARTMENTS OF RETROPERITINEUM
It is divided by fascial planes into three
compartments
• Anterior pararenal space
• Perirenal (or perinephric) space
• Posterior pararenal space
5. ANTERIOR PARARENAL SPACE
Anterior pararenal space contains
• retroperitoneal portions of the colon,
duodenum, and pancreas
• continuous with the transverse mesocolon
and root of the small bowel mesentery,
• confined by the posterior parietal peritoneum
anteriorly, the anterior renal fascia posteriorly,
and the lateroconal fascia laterally
6.
7. PERIRENAL SPACE
• The perirenal space contains the kidney, renal
vessels, adrenal gland, renal pelvis, proximal
ureter, perirenal lymphatics, and perirenal fat.
• Confined by the anterior renal fascia and
posterior renal fascia
8.
9. POSTERIOR PARARENAL SPACE
• Posterior pararenal space almost always contains
only fat.
• posterior renal fascia anteriorly,
• transversalis fascia posteriorly,
• psoas muscle medially,
• continues laterally external to the lateroconal
fascia as the properitoneal fat of the abdominal
wall.
• Inferiorly the posterior pararenal space is open to
the pelvis
• superiorly it continues as a thin subdiaphragmatic
layer of extraperitoneal fat.
10. RETROPERITONEAL ANATOMY: PLANES
• Lateroconal planes are relatively small. They extend down the length of the
ascending and descending colon just lateral to the colonic lumen.
• Retromesenteric planes are major routes by which effusions are distributed
in the extraperitoneal abdomen.
• The route of escape for common pathologic entities, such as pancreatitis and
colitis.
11.
12. RETROPERITONEAL ANATOMY: PLANES
• Retrorenal plane extends inferior to the cone of renal fascia, it joins the
retromesenteric plane to become a combined plane that is situated on the
anterolateral surface of the psoas muscle, lateral to the ureter and medial to
the iliac vessels. It continues into the pelvic extraperitoneum.
16. RETROPERITONEAL SARCOMA
• Rare mesenchymal neoplasms, accounting for less than 1% of
adult malignancies.
• 15% of sarcomas originate within the retroperitoneum.
• May develop at any age, but most present in the sixth and seventh
decades of life.
• More commonly in men
17. There are approximately 50 histologic subtypes of soft tissue
sarcoma.
•Liposarcoma (40%)
•Leiomyosarcoma (30%)
•Undifferentiated pleomorphic sarcoma (UPS) (previously known
as malignant fibrous histiocytoma) (15%).
18. LIPOSARCOMA
• Most common retroperitoneal sarcoma
• Originate from primitive mesenchymal cells
• World Health Organization (WHO) divides liposarcomas into four subtypes:
• Well-differentiated
• Myxoid
• Dedifferentiated
• Pleomorphic
19. RADIOGRAPHIC FEATURES
CT
• Heterogenous mass
• Varying amount of fat and soft tissue
• Multiple septae
• Enhancing soft tissue components
MRI
• Myxoid - hyperintense on T2WI with delayed post-contrast
enhancement
20. Axial contrast-enhanced CT
image demonstrates large
fat-attenuation mass
involving right perirenal
space causing anterior
displacement of right
kidney and IVC .
21. • Axial contrast-enhanced CT
image through abdomen shows
large heterogeneously
enhancing retroperitoneal
mass with predominantly
fatty attenuation
22. MRI images demonstrate a large heterogeneously enhancing mass
with internal fat, compatible with a retroperitoneal liposarcoma
23. LEIOMYOSARCOMA
• Retroperitoneum is considered the most common extra uterine site for
leiomyosarcoma.
• Symptoms are often non-specific with abdominal pain being on of the most
frequent complaints.
• They arise from smooth muscle within arteries, veins or bowel. The
most frequent site is the IVC (50%), and they are more common in women.
They can be:
• completely extra-vascular: 62%
• completely intra-vascular: 5%
• have extra- and intra-luminal components: 33%
24. • Retroperitoneal leiomyosarcomas tend to develop massive cystic
components which undergo degeneration. Unlike other sarcomas, fat and
calcification are not typically present.
• MRI
• T1: intermediate to low signal intensity
• T2: intermediate to high signal intensity
25. UNDIFFERENTIATED
PLEOMORPHIC SARCOMA
• UPS, previously known as malignant fibrous histiocytoma
• 15% of all malignant fibrous histiocytomas occur within the retroperitoneum.
• Six histological subtypes:
• Storiform
• Pleomorphic
• Myxoid
• Giant cell
• Inflammatory
• Angiomatoid
26. CT
• The density of MFH is typically similar
to adjacent muscle, with heterogeneous
lower density areas if haemorrhage,
necrosis or myxoid material is abundant.
• The soft tissue component enhances.
Malignant Fibrous Histiocytoma (MFH) - Soft tissue mass that
appears to arise from the left kidney but closer inspection reveals a
cleft fat plane between the left kidney and retroperitoneal mass
lesion
27. MRI
MRI is the modality of choice for assessing soft tissue sarcomas.
These tumours are typically relatively well circumscribed, located
within or adjacent to muscle, exerting positive mass effect on
surrounding structures due to their (usual) large size at
presentation.
T1
intermediate (to low) signal intensity, similar to adjacent
muscle 3-4
prominent enhancement of solid components
T2
intermediate to high signal intensity
29. PARAGANGLIOMA
• a/k/a extraadrenal pheochromocytomas.
• Arise from highly vascularized specialized neural crest cells called
paraganglia that are symmetrically distributed along the aortic axis in close
association with the sympathetic chain in the neck, chest, abdomen, and
pelvis.
• Fourth and fifth decades of life
• Men and women are affected equally
30. • Paragangliomas may be considered to be functional or
nonfunctional depending on whether catecholamines are secreted
or not.
• Complete surgical resection is the treatment of choice for
abdominal paraganglioma; adjunctive therapies including
radiotherapy and chemotherapy are considered palliative
31. CT
• Enhancing, well-circumscribed, lobular or round soft tissue–attenuation
masses
• Homogeneous in attenuation when small or heterogeneous when
large.
• Central areas of low attenuation may be due to central necrosis or cystic
change.
• Punctate calcification and focal areas of high attenuation due to acute
hemorrhage may also be seen in some tumors
32. MRI
• SI and enhancement characteristics of paragangliomas are similar to those of
adrenal pheochromocytoma.
• Low to intermediate SI on T1-weighted images
• Moderately high SI on T2 relative to skeletal muscle
• Commonly heterogeneous secondary to foci of intratumoral necrosis or
hemorrhage
33. Coronal T1-weighted and T2-weighted images show well-circumscribed retroperitoneal mass (M) that abuts right lobe
of liver (L) and pancreatic head (P), with heterogeneous low to intermediate T1 SI and intermediate to high T2 SI
relative to skeletal muscle
34. NEUROFIBROMA
• Benign tumors of nerve sheaths of peripheral nerves
• Commonly occur in deep anatomic locations in patients with NF1
(especially in retroperitoneal and paraspinal locations) and are commonly
associated with neurologic symptoms.
• Neurofibromas are not encapsulated.
• Localized, plexiform, or diffuse types.
35. When multiple or of the plexiform type, which is pathognomonic of NF1,
large conglomerate infiltrative masses of innumerable neurofibromas
diffusely thicken a parent nerve and extend into multiple nerve branches,
resulting in a characteristic “bag of worms” appearance
36. Known case of
neurofibromatosis type I:
Axial T2W fat-saturated
images show multiple
hyperintense lesions in
the pelvis, with a few of
them showing central
hypointensity suggestive
of a ‘Target sign’
37. Extraperitoneal pelvic neurofibromas in 33-year-old woman with
neurofibromatosis type 1, back pain, and right lower extremity
pain. A to C, Axial T1-weighted (A), axial fat-suppressed T2-
weighted (B), and coronal T2-weighted short tau inversion
recovery (STIR) (C) images through pelvis demonstrate multiple
well-circumscribed masses (arrows) in left ischiorectal fossa, right
inguinal region, and right posterior pelvis. Masses have homogeneous low T1
SI and heterogeneous high T2 SI relative to skeletal muscle, with curvilinear
areas of low T2 SI resulting in whorled appearance
• CT- low soft tissue attenuation
• MRI- low SI on T1 and high SI on T2
38. SCHWANNOMA
• Schwannoma, or neurilemoma, is a benign tumor that arises from the
perineural sheath of Schwann (neurilemma).
• Schwannoma accounts for 6% of retroperitoneal neoplasms and is more
common than neurofibroma.
• Asymptomatic
• Females (2:1)
• 20–50-year age group
39. CT - small schwannomas are round, well defined, and homogeneous, but
large schwannomas may be heterogeneous in appearance.
Calcification can be punctate, mottled, or curvilinear.
The nerve of origin is often difficult to identify.
After contrast enhancement, schwannoma demonstrates variable
homogeneous or heterogeneous enhancement
40. Axial contrast-enhanced CT
show round well-
circumscribed retroperitoneal
mass with predominantly low
attenuation and scattered
regions of soft tissue
attenuation.
41. Axial T1-weighted and T2-weighted images through pelvis show round well-circumscribed retroperitoneal mass with
slightly low to intermediate T1-weighted SI, high T2-weighted SI relative to skeletal muscle with enhancing regions of
lower T2-weighted SI,.
42. • “split-fat sign” may be seen when a rim of fat surrounds the tumor,
originating from a nerve in an intermuscular location.
Schwannoma with a split fat
sign: T1W and T2W sagittal
images show an oblongated
encapsulated lesion, which is
isointense on T1 and
hyperintense on T2 on the
posterior aspect of the arm in
the intermuscular plane, with
preserved fat at the upper and
lower ends.
43. GANGLIONEUROMA
• rare benign tumor that arises from the sympathetic ganglia.
• The retroperitoneum (32%–52% of cases) and mediastinum (39%– 43% of
cases) are the most common sites for ganglioneuroma, followed by the
cervical region (8%–9% of cases).
• In the retroperitoneum, the tumor is commonly seen along the paravertebral
sympathetic ganglia (59% of cases) or, less commonly, in the adrenal
medulla.
44. • CT- circumscribed lobulated hypoattenuating mass that may surround a
blood vessel without narrowing the lumen. Discrete punctate calcifications
are seen in 20%–30%
Axial contrast-enhanced CT
images through pelvis demonstrate
infiltrative lobulated pelvic
extraperitoneal mass with low
attenuation
45. GANGLIONEUROBLASTOMA
• Ganglioneuroblastoma is an intermediate-grade tumor that has
elements of benign ganglioneuroma and malignant
neuroblastoma.
• Ganglioneuroblastoma is a pediatric tumor occurring in the 2–4-
year age group.
• Imaging appearances vary, and the tumor could be solid or cystic
with solid components.
46. NEUROBLASTOMA
• Malignant
• More commonly seen in males and in the 1st decade of life.
• Two-thirds of neuroblastomas are located in the adrenal gland,
and the remaining neuroblastomas occur along the paravertebral
sympathetic chain.
47. At CT and MR imaging,
neuroblastoma is irregular, lobulated,
and heterogeneous and demonstrates
coarse amorphous calcifications and
variable contrast enhancement, as well
as invasion of adjacent organs and
encasement of vessels with luminal
compression.
49. FIBROMATOSES
The fibromatoses are divided into
• Superficial (fascial) groups
• Deep (desmoid tumor) groups - further subdivided into
Extraabdominal,
Abdominal wall
Intraabdominal (mesenteric, mesocolic, omental, retroperitoneal) subgroups
50. CT and MRI
• Desmoid tumors are typically infiltrative and cross fascial boundaries
• Homogeneous low to intermediate attenuation
• Low to intermediate SI on T1- and t2-weighted images relative to skeletal muscle,
• Tend to appear aggressive although they are not malignant.
51. Axial T1-weighted (A), T2-weighted (B), images through pelvis show infiltrative left pelvic mass with intermediate T1
and intermediate to slightly high T2 SI relative to skeletal muscle, and marked enhancement. Mass infiltrates left
bladder wall, left obturator internus muscle, left rectus abdominis muscle, and proximal left thigh musculature, with
encasement of left common femoral artery and compression of left common femoral vein.
52. RETROPERITONEAL FIBROSIS (RPF)
• Retroperitoneal fibrosis is an uncommon fibrotic reaction in
the retroperitoneum that typically presents with ureteric obstruction.
• Idiopathic retroperitoneal fibrosis (IRF)
• Perianeurysmal retroperitoneal fibrosis
• Isolated periaortitis: corresponds to a non-aneurysmal form of
chronic periaortitis
• Inflammatory abdominal aortic aneurysm (IAAA)
53. CT
• Retroperitoneal fibrosis is visible as a soft tissue density mass located around
the aorta and iliac arteries. Classically, it develops around the aortic
bifurcation and spreads upwards where it can envelop the renal hila.
• Early or active stages - variable enhancement can be seen with intravenous
contrast
• Quiescent disease - no enhancement
55. LIPOMA
• Lipomas are benign mesenchymal tumors composed of mature fat and
represent the most common mesenchymal neoplasm.
• They are the most common benign tumor of the retroperitoneum
Angiolipoma
Myolipoma
Angiomyolipoma
Myelolipoma
Spindle cell/pleomorphic lipoma
Lipoblastoma
56. • CT - lipomas typically have homogeneous attenuation
• MRI - SI identical to that of macroscopic fat on all pulse
sequences, without enhancing components.
• Few thin (<2 mm) septae may be seen that have minimal to
moderate enhancement
• Mild enhancement of a thin fibrous capsule
57. ANGIOMYOLIPOMA
• Angiomyolipomas (AMLs) are benign neoplasms composed of ectopic
rests of normal tissue in an abnormal arrangement containing fat,
abnormal blood vessels, and smooth muscle in varying relative
proportions and are most often found in the kidneys.
• 90% of AMLs occur sporadically in the kidney
58. • On CT and MRI, AMLs typically
appear as masses that contain areas of
macroscopic fat. Soft tissue
components and vascular components
are frequently present, and there is a
variable amount of enhancement
60. RETROPERITONEAL FLUID
COLLECTION
• Retroperitoneal fluid collections may result from neoplastic, infectious, inflammatory, and
traumatic causes.
• Fascial planes or adhesions tend to confine retroperitoneal fluid collections to their
compartment of origin
• Simple fluid collections and transudative fluid collections - water attenuation, low
SI on T1-weighted images, and very high SI on T2-weighted images.
• Proteinaceous fluid collections and exudative fluid collections - greater attenuation
than water, variably increased SI on T1, and variably decreased SI on T2 .
61. HEMORRHAGE/HEMATOMA
• Retroperitoneal hemorrhage may be secondary to rupture or leak of an
aneurysm or vascular malformation, anticoagulant therapy, a bleeding
diathesis, trauma, arterial catheterization, underlying parenchymal organ
pathology with rupture, or may be spontaneous.
• Retroperitoneal hematoma volume (≥1600 mL) is an independent
predictor of decreased overall survival.
62. CT - HAEMATOMA
• Acute clotted hemorrhage -high attenuation (45-70 HU )
• Acute non clotted hemorrhage or more chronic hemorrhage - lower attenuation
(20-45 HU) or fluid attenuation (0-20 HU)
• Sentinel clot sign, where areas of higher-attenuation acute clotted hemorrhage
are more likely to indicate anatomic sites of hemorrhage
63. MRI- HAEMATOMA
• On MRI the appearance of hemorrhage is dependent upon on its age
Axial fat-suppressed T1-weighted
image shows well-defined lesion
anterior to right iliac bone that has
central intermediate SI and peripheral
high-SI rim (due to subacute
methemoglobin) relative to skeletal
muscle. Note anterior displacement of
iliopsoas muscle
64.
65. CONCENTRIC RING SIGN
Subacute hematoma may show two outer characteristic layers of SI
• Thin peripheral rim - low SI on all pulse sequences- hemosiderin,
• Inner rim - high-SI zone on T1-weighted images - methemoglobin.
• Central component of the hematoma - variable si - based on the amounts
of seroma, deoxyhemoglobin, and methemoglobin present.
This appearance of the periphery of a fluid collection on T1 is known as the
concentric ring sign and is pathognomonic for a subacute hematoma.
66. • Hematocrit effect with fluid-fluid levels due to layering of heavier cellular
elements of blood located dependent to liquid supernatant may be seen on
CT or MRI.
• Active arterial hemorrhage is recognized as a focus or jet of very high
attenuation (mean, ≈130 hu) or high si on contrast-enhanced ct and t1-
weighted mri.
• When active arterial hemorrhage is present within a liquefied hematoma that
has a hematocrit effect, a signal flare phenomenon
67. LYMPHOCELES
• Lymphoceles are fluid-filled cystic lesions that usually occur at least 3 to 4
weeks after pelvic or retroperitoneal lymphadenectomy for urologic or
gynecologic malignancies or after renal transplantation .
• Small lymphoceles - asymptomatic and spontaneously resorb.
• Large retroperitoneal lymphoceles - symptomatic and may lead to
complications related to mass effect upon adjacent structure
68.
69. URINIFEROUS FLUID/URINOMA
• Most commonly within the perirenal spaces.
• CAUSE - Due to urinary obstruction but may be due to
abdominopelvic trauma, surgery, or diagnostic instrumentation.
• As urine extravasates into the retroperitoneum, it can cause
lipolysis of the surrounding fat, with resultant encapsulation of
urine, forming a urinoma.
70.
71. PSEUDOMYXOMA RETROPERITONEI
• Benign or malignant condition that results in intraperitoneal mucin
accumulation.
Pseudomyxoma retroperitonei occurs very rarely
Caused by retroperitoneal rupture of a primary appendiceal mucinous
adenoma in a retrocecal appendix and fixation of the lesion to the posterior
abdominal wall.
72. • CT - multicystic lesions with low attenuation
• MRI - low to intermediate SI on T1-weighted images, and high SI on T2-
weighted images
• Thickened walls or septations that displace and distort adjacent structures.
73. PNEUMORETROPERITONEUM
• Retroperitoneal gas is most often the result of bowel perforation secondary
to trauma, iatrogenic causes such as surgery or endoscopy, inflammatory
bowel disease, peptic ulcer disease, foreign body, or gas-producing
infection.
• On CT and MRI, retroperitoneal gas has very low attenuation and very
low SI on T1- and T2-weighted images and may either be punctate or
confluent, sometimes with air-fluid levels
76. 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.
77. 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.
78. RETROPERITONEAL FIBROSIS
• Retroperitoneal fibrosis is an uncommon collagen vascular disease of unknown cause that
can mimic a retroperitoneal tumor.
• Typically idiopathic (>70% of cases) and is likely autoimmune in origin.
• 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
79. • CT - irregular plaquelike 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.
80. CT
• Avid enhancement - active stages of retroperitoneal fibrosis
• Little or no enhancement - chronic phase of retroperitoneal fibrosis
MR
• High signal intensity on T2 WI in the acute phase of the disease,
with early contrast enhancement
• Low signal intensity in the chronic fibrosing phase, with delayed
enhancement.
81. ERDHEIM-CHESTER DISEASE
• Rare non-Langerhans form of
histiocytosis of unknown origin.
• 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.
82. CYSTIC NEOPLASTIC MASSES
Cystic Change in Solid Neoplasms
Cystic changes may develop in solid lesions such as paragangliomas and
neurilemomas
Cystic Teratoma
CT shows a complex cystic mass, with areas of fat and calcification.
Lymphangioma
Lymphangioma is a developmental malformation that is caused by failure
of communication of retroperitoneal lymphatic tissue with the main
lymphatic vessels.
83. LYMPHANGIOMA
• Lymphangioma can be seen in perirenal, pararenal, or pelvic
extraperitoneal spaces and can involve more than one
compartment.
• CT - large thin-walled unilocular or multilocular cystic mass
with attenuation values ranging from that of fat (caused by
chyle) to that of fluid.
• Calcification is rarely seen.
84. • An elongated shape and the involvement of multiple compartments are
salient features of lymphangiomas
• Prone to recurrence.
• The occurrence of septa, compression of intestinal loops, and a lack of
fluid in dependent recesses and mesenteric leaves differentiate
lymphangioma from ascites.
85. Axial T1-weighted and fat-suppressed T2-weighted images show well-circumscribed lobulated retroperitoneal lesion
with low T1 SI and very high T2 SI relative to skeletal muscle, with several thin low-T2-SI internal septations. Note
portion of lesion that invaginates between abdominal aorta and lumbar spine , which is characteristic of lymphangioma.
