1. Overview of Retroperitoneal Anatomy, Masses and Disease Spread Imaging
Saeed Elojeimy MD-Phd 1
Mariam Moshiri MD 1
Puneet Bhargava MD2
Sheriff Osman MD 1
Charles A Rohrmann MD 1
1 Department of Radiology, University of Washington Medical Center, Seattle WA
2 Department of Radiology, University of Washington & VA Puget Sound Health Care System, Seattle WA

2. Pretest unknown 1: can you name the fascial layer marked with the arrow ?

3. Pretest unknown 2 : what is the diagnosis ?

4. Pretest unknown 3 : what is the diagnosis ?
MRI in phase
MRI out of phase

5. Pretest unknown 4 : what is the diagnosis ?

6. Goals and Objectives
1. Review anatomy of the retroperitoneum
2. Illustrate pathways for disease spread in the retroperitoneum
3. Present an algorithmic approach for the differential diagnosis of retroperitoneal masses
4. Review pathognomonic findings of syndromes with RP involvement

7. Introduction to RP Anatomy
Borders: bordered anteriorly by peritoneal fasciae and posteriorly by the posterior continuation of the transversalis fascia (marked by thick blue line).
Transversalis fascia
Contents: ascending and descending colon, duodenum, pancreas, aorta, IVC, kidneys, adrenal gland, proximal ureters, and retroperitoneal fat.
Posterior renal fascia
Anterior renal fascia
Lateroconal fascia
Peritoneal fasciae

8. Anterior renal fascia (ARF) aka Gerota’s fascia
RP anatomy
RP fasciae
Anterior renal fascia (ARF) aka Gerota’s fascia
Posterior renal fascia (PRF) aka Zuckerkandl’s fascia
Lateroconal fascia
(LCF)
ARF
LCF
PRF

9. Retromesenteric: between layers of anterior renal fascia
RP anatomy
Interfascial planes
Retromesenteric: between layers of anterior renal fascia
Retrorenal: Between layers of posterior renal fascia
Lateroconal: between layers of lateroconal fascia
Lateroconal
plane
Retromesenteric
Retrorenal plane

10. RP anatomy A B C
Axial CT images (A-C) showing progressive thickening and fluid dilatation of the retrorenal fascial plane (yellow arrows) secondary to inflammation/ infection. Note fluid extension in image C into the lateroconal plane (blue arrows).

11. Anterior pararenal space Perirenal space Posterior para-renal space
RP anatomy
RP spaces
Anterior pararenal space
Perirenal space
Posterior para-renal space

12. Anterior pararenal space (green highlight)
RP anatomy
RP spaces
Anterior pararenal space (green highlight)
Borders: between peritoneum and ARF
Contents: ascending and descending colon, duodenum, and pancreas (Think GI)

13. Perirenal space (purple highlight)
RP anatomy
RP spaces
Perirenal space (purple highlight)
Borders: between ARF and PRF
Contents: kidneys, adrenal glands, proximal ureters, perirenal fat, lymphatic vessels, and blood vessels (Think GU)

14. Posterior para-renal space (blue highlight)
RP anatomy
RP spaces
Posterior para-renal space (blue highlight)
Borders: between PRF and transversalis fascia
Contents: predominantly fat

15. RP anatomy A B C
A and B: Perirenal fluid (yellow arrow) secondary to calyceal rupture in a patient with right ureteric stone (blue arrow). Note lack of hydronephrosis, consistent with relief of obstruction secondary to calyceal rupture. C: Perirenal stranding in a patient with pyelonephritis (orange arrows).

16. Pathways for disease spread
Direct extension
Lymphatic spread

17. Pathways for disease spread
Direct extension
Disease can spread within the RP by dissecting through the interfascial planes (see examples in the following slides). In addition, The strategic location of the RP makes it a pathway for disease spread among the surrounding compartments including the pelvis, peritoneum and mediastinum as illustrated in the diagram on the right.
Mediastinum
RP spaces
Peritoneum
Pelvis

18. Direct extension A B C
Axial CECT images (A-C) showing a case of sigmoid rupture with air dissecting from pelvis (yellow arrows) into the RP (blue arrows) .

19. * * * * Direct extension A B
Axial CT showing acute pancreatitis with fluid and gas bubbles in anterior pararenal space (*) dissecting through retrorenal interfascial plane (blue arrow), and extending into posterior RP space (yellow arrow).
Coronal CT in a patient with a psoas abscess showing extension of infection and inflammatory fluid into the posterior pararenal spaces and interfascial planes (blue arrows).

20. * * * * * * * * Direct extension A B C D
Axial CECT (A-D) illustrating a case of acute pancreatitis with fluid in anterior pararenal space (*) extending anteriorly into the peritoneum (yellow arrow) and inferiorly into the pelvis (blue arrow).

21. Pathways for disease spread
Lymphatic spread
Mediastinal and pleural lymph nodes
Within the RP, the perinephric bridging septae connect the kidneys to pararenal lymphatics which are inter-connected among themselves.
The aorto-caval lymph nodes act as a bridge between RP lymphatics and lymph nodes in the chest and pelvis as illustrated in diagram on the left.
Aorto-caval Lymph nodes
Perirenal and pararenal
lymphatics
Pelvic and inguinal lymph nodes

22. Lymphatic spread A D B C
Axial (A-C) and coronal (D) CT images showing a case of lymphangiomatosis extending through lymphatics of the RP (blue arrows) and involving lymphatics of the posterior mediastinum (yellow arrows) and pelvis (orange arrows). Tiny calcifications/ phleboliths are related to additional hemangioma component.

23. Overview of RP masses Renal Adrenal Pancreatic Other
General recommendations for approaching retroperitoneal masses:
Determine organ of origin.
Look for tissue composition : cystic vs predominantly fatty vs soft tissue.
Look for other features: +/- calcifications, enhancement compared to organ of origin.
History and other clinical info: recent surgery, trauma.

24. Renal masses Cystic Solid Bosniak classification1
Is there macroscopic fat ?
Simplified
Bosniak
Simple/ Benign
Indeterminate/ Malignant
Septa/ wall
Thin
Thick
Ca++
None/fine
Enhancing
Nodule
None
Yes No
Yes
-Renal cell carcinoma (RCC)2
-Transitional cell carcinoma
-Lymphoma
-Metastasis
-Angiomyolipoma
1.To learn more about Bosniak classification, click on following link : Bosniak renal cyst classification | Radiology Reference Article | Radiopaedia.org
2. Note that RCC can rarely contain macroscopic fat.

25. Renal masses A B C
A. Coronal CT showing a simple bosniak type 1 renal cyst (yellow arrow) B. Coronal CT showing a Bosniak type 3 renal cyst (blue arrows) with thick coarse calcification within wall (blue arrowhead) C. Coronal CT showing bilateral renal cysts in a patient with adult polycystic kidney disease (orange arrows)

26. Renal masses A B C
A. Axial CT showing a heterogeneous exophytic soft tissue renal mass consistent with RCC (yellow arrows) B. Axial CT showing a homogeneous large renal soft tissue mass consistent with lymphoma (blue arrows). C. Axial CT showing a fat containing renal mass consistent with angiomyolipoma.

27. Adrenal masses Cystic Solid Is there macroscopic fat ? No Yes
-Adrenal cyst
-Adrenal hematoma
Is there macroscopic fat ? No
Yes
Is there microscopic fat ?*
-Myelolipoma No
Yes
-Metastasis
-Adrenocortical carcinoma
-Pheochromocytoma
-Lymphoma
-Adenoma
* To learn more about microscopic fat and its detection,refer to the following article :CT and MR Imaging of Extrahepatic Fatty Masses of the Abdomen and Pelvis: Techniques, Diagnosis, Differential Diagnosis, and Pitfalls1

28. Adrenal masses A B C
A. Axial CT showing a right adrenal mass in a patient following MVC that resolved on follow-up imaging (not shown) consistent with an adrenal hematoma (yellow arrows) B. Axial CT showing a low density left adrenal cystic structure consistent with an adrenal cyst (blue arrows) C. Coronal CT showing a fat containing adrenal mass consistent with myelolipoma (orange arrows).

29. Adrenal masses A B C
A. Axial MRI images showing a right adrenal mass with high signal on in phase sequence (top, black arrows) that loses signal on out of phase sequence (bottom, yellow arrows) consistent with adrenal adenoma B. Axial CT of a patient with lung cancer showing a left adrenal metastasis (blue arrows) C. Coronal CT showing a case of lymphoma involving the right adrenal gland (orange arrowheads) and right kidney (orange arrows).

30. Lesion enhancement more or less compared to normal pancreatic tissue ?
Pancreatic masses
Cystic
Solid
-Pancreatic pseudocyst
-Microcystic serous adenoma
-Mucinous cystic neplasm
-Solitary and papillary epithelial neoplasm (SPEN)
-Intraductal papillary mucinous tumor (IPMN)
-Other: Cystic mets, lymphoma
Lesion enhancement more or less compared to normal pancreatic tissue ?
Less
More
-Pancreatic adenocarcinoma
-Metastasis
-Pheochromocytoma
-Islet cell tumors (insulinoma, gastrinoma)
-Metastasis

31. Pancreatic masses B A C
A. Axial CT image showing tiny cysts in pancreatic head characteristic of microcystic serous adenoma (blue arrowheads). B. Axial CT image showing an islet cell tumor (yellow arrows), hyperenhancing compared to normal pancreatic tissue (yellow arrowheads). C. Axial CT image a pancreatic adenocarcinoma (orange arrows), hypoenhancing compared to normal pancreatic tissue (orange arrowheads).

32. Pancreatic masses *
Axial (right) and coronal (left) CT images showing dilated pancreatic (yellow arrows) and hepatic (black arrows) ducts (double duct sign) caused by a pancreatic pseudocyst (marked with a star). Note scatterered pancreatic calcifications (yellow arrowheads, right and left images) consistent with chronic pancreatitis.

33. Primary RP masses Cystic Solid Does mass contain fat? No Yes
-Lymphocele
-Seroma
-Pseudocyst
-Abscess
-Hematoma
-Urinoma
-Lymphangioma
Does mass contain fat? No
Yes
-Mesoderm origin: leiomyoma/ sarcoma, lymphoma, RP fibrosis (lymphoma mimic).
-Germ Cell tumors
-Neurogenic and nerve sheath tumors: neurofibroma, neuroblastoma.
-Metastasis
-Lipoma
-Liposarcoma
-Teratoma

34. Primary RP masses A C B
A. Axial CT image in a patient post MVC showing a perirenal hematoma (yellow arrows) with active extravasation (yellow arrowheads). B. Axial CT showing a post-surgical lymphocele (blue arrows). Note surgical clips (blue arrowheads). C. Coronal CT showing a leiomyosarcoma (orange arrows) with IVC invasion (orange arrowheads).

35. Other RP masses A B C
A. Axial CT images showing a case of RP fibrosis (yellow arrows), a mimic for RP lymphoma. Lymphoma tends to displace aorta (yellow arrowheads) anteriorly, a differentiating feature not seen with RP fibrosis. B. Axial CT showing a RP soft tissue mass with multiple cystic regions and calcifications (arrow-heads) consistent with a teratoma. C. Axial CT large predominantly fat containing RP mass (orange arrow-heads) consistent with liposarcoma.

36. Pathognomonic Findings of Syndromes with RP Involvement?
Birt-Hogg- Dube
Left renal cancer Pulmonary cysts
Erdheim-Chester ?
Peri-renal fibrosis
Interlobular septal thickening

37. ? RP syndromes Bilateral AML Hepatic AML TuberousSclerosis Lung cysts
Cortical Tubers

38. RP syndromes
Lymphangiomatosis
Splenic lesions ?
Bony lytic lesions 38

39. Summary and Review Slides

40. Summary
1. We reviewed retroperitoneal anatomy including retroperitoneal spaces, fasciae and interfascial planes.
2. We illustrated examples of retroperitoneal spaces and planes connecting to each other, and with other spaces in chest abdomen and pelvis.
3. We provided an algorithmic approach for approaching retroperitoneal masses.
4. We reviewed pathognomonic findings of syndromes that involve the retroperitoneum.

41. Retroperitoneum Anatomy: Rule of 3’s
I. Three fasciae
II. Three spaces
III. Three interfascial planes*
1. Anterior renal fascia (ARF)
1. Anterior pararenal space (APS)
1. Retromesenteric plane (RMP)
2. Lateroconal fascia (LCF)
2. Perirenal space (PRS)
2. Lateroconal plane (LCP)
3. Posterior renal fascia (PRF)
3. Posterior pararenal space (PPS)
3. Retrorenal plane (RRP)
APS
ARF/RMP
PRS
LCF/LCP
PPS
PRF/RRP
* Note that RP interfascial planes represent pathologic dilatation of corresponding RP fasciae .

42. Pathways for Disease Spread
Direct Extension
Lymphatic Spread
RP Spaces
Mediastinum
Pelvis
Peritoneum
Chest LNs
Aorto-caval LNs
Pararenal LNS
Pelvic and inguinal LNs

43. Summarized Algorithm for Approaching RP masses
Renal
Adrenal
Pancreatic
Primary RP
Soft tissue
Cystic
Soft tissue
Cystic
Soft tissue
Cystic
Soft tissue
Cystic
Simple cyst
Cyst
Hematoma
Pseudocyst
Mucinous and serous cystic neplasms
SPEN
IPMN
Cystic metastasis
Lymphoma
Lymphocele
Seroma
Pseudocyst
Abscess
Hematoma
Urinoma
Lymphangioma
Metastasis
Adrenocortical carcinoma
Pheo-chromocytoma
Lymphoma 1 3 4 1
Mesoderm tumors
Germ cell tumors
Neurogenic tumors
Metastasis 1 2
RCC
TCC
Lymphoma
Metastasis
Pheo-chromocytoma
Islet cell tumors
Metastasis
Adeno-carcinoma
Metastasis
Lipoma
Liposarcoma
Teratoma
AML
Myelolipoma
Adenoma
1. Contains macroscopic fat 2. Contains microscopic fat 3. Hyperenhancing compared to pancreatic tissue
4. Hypoenhancing compared to pancreatic tissue

44. Post-test unknown 1: can you name the fascial layer marked with the arrow ?

45. Unknown 1 answer: posterior renal fascia

46. Post-test unknown 2 : what is the diagnosis ?

47. Unknown 2 answer: lymphangioma

48. Post-test unknown 3 : what is the diagnosis ?
MRI in phase
MRI out of phase

49. Unknown 3 answer: adrenal adenoma
MRI in phase
MRI out of phase

50. Post-test unknown 4 : what is the diagnosis ?

51. Unknown 4 answer: Adult Polycystic Kidney Disease

52. References
1.Simons, G.W., J.R. Sty, and R.J. Starshak, Retroperitoneal and retrofascial abscesses. A review. J Bone Joint Surg Am, (8): p
2.Yang, D.M., et al., Retroperitoneal cystic masses: CT, clinical, and pathologic findings and literature review. Radiographics, (5): p
3.Shin, N.Y., et al., The differential imaging features of fat-containing tumors in the peritoneal cavity and retroperitoneum: the radiologic-pathologic correlation. Korean J Radiol, (3): p
4.Aizenstein, R.I., A.C. Wilbur, and H.K. O'Neil, Interfascial and perinephric pathways in the spread of retroperitoneal disease: refined concepts based on CT observations. AJR Am J Roentgenol, (3): p
5.Nishimura, H., et al., MR imaging of soft-tissue masses of the extraperitoneal spaces. Radiographics, (5): p
6.Rajiah, P., et al., Imaging of uncommon retroperitoneal masses. Radiographics, (4): p
7. Stat DX
8. Radiopedia