Retroperitoneum

Radiology of Retroperitoneum - Basics

1. RETROPERITONEUM - Dr.N.Suriyaprakash

2. RETROPERITONEUM The retroperitoneum is the compartmentalized space located external to and predominantly posterior to the posterior parietal peritoneum.

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. 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

7. 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.

8. 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.

9. 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 .

10. Retroperitoneal sarcoma Retroperitoneal neurogenic tumors Retroperitoneal fat-containing lesions Solid non- neoplastic masses Retroperitoneal fluid collection Pneumoretroperitoneum Cystic neoplastic masses

11. 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. M ore commonly in men

12. 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%).

13. LIPOSARCOMA Most common retroperitoneal sarcoma Originate from primitive mesenchymal cells World Health Organization (WHO) divides liposarcomas into four subtypes: Well-differentiated Myxoid Dedifferentiated Pleomorphic

14. 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

15. Axial contrast-enhanced CT image demonstrates large fat-attenuation mass involving right perirenal space causing anterior displacement of right kidney and IVC .

16. Axial contrast-enhanced CT image through abdomen shows large heterogeneously enhancing retroperitoneal mass with predominantly fatty attenuation

17. MRI images demonstrate a large heterogeneously enhancing mass with internal fat, compatible with a retroperitoneal liposarcoma

18. 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%

19. 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

20. 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

21. 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

22. 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

23. RETROPERITONEAL NEUROGENIC TUMORS Paraganglioma Ganglioneuroma Neuroblastoma Ganglioneuroblastoma Schwannoma Neurofibroma Malignant nerve sheath tumour

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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.

30. 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

31. 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’

32. 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

33. 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

34. 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

35. Axial contrast-enhanced CT show round well-circumscribed retroperitoneal mass with predominantly low attenuation and scattered regions of soft tissue attenuation.

36. 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,.

37. “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.

38. 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.

39. 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

40. 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.

41. 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.

42. 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 .

43. RETROPERITONEAL FIBROTIC LESIONS Retroperitoneal Fibrosis Inflammatory Abdominal Aortic Aneurysm Fibromatosis ( Desmoid Tumor )

44. 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

45. 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.

46. 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.

47. 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)

48. 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

49. PRIMARY RETROPERITONEAL FAT-CONTAINING LESIONS

50. 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

51. CT - lipomas typically have homogeneous attenuation MRI - SI identical to that of macroscopic f at 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

52. 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

53. 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

54. RETROPERITONEAL FLUID COLLECTION Haematoma Uriniferous Fluid/ Urinoma Lymphocele Chyloretroperitoneum Biloma Pseudomyxoma Retroperitonei Inflammatory Noninfectious and Infectious Fluid Collections Hydatid Cyst

55. 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 .

56. 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 .

57. 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

58. 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

59. 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.

60. 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

61. 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

62. 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 .

63. 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.

64. 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.

65. 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

66. SOLID NON-NEOPLASTIC MASSES 1. Pseudotumoral lipomatosis 2. Retroperitoneal fibrosis 3. Erdheim -Chester disease 4. Extramedullary hematopoiesis

67. 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.

68. 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.

69. 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

70. 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.

71. 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.

72. 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.

73. 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.

74. 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.

75. 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.

76. 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 .