The retroperitoneum is positioned posterior to the parietal peritoneum and anterior to the transversalis fascia. (1) It has three distinct components: the anterior pararenal space, the perirenal space, and the posterior pararenal space. (1,2) The anterior pararenal space contains pancreas, 2nd to 4th parts of the duodenum, and the ascending and descending colon. The perirenal space contains the kidneys, proximal ureters, adrenal glands, and perirenal fat, while the posterior pararenal space contains adipose tissue and connects inferiorly to the pelvic extraperitoneal space. The majority of retroperitoneal masses arise from retroperitoneal organs and are not classified as primary retroperitoneal masses. A primary retroperitoneal mass is identified when situated within the retroperitoneal space and after ruling out its origin from an organ. These masses can be classified into solid and cystic types, further classified as neoplastic and non-neoplastic subtypes. Primary retroperitoneal cysts, which are exceedingly rare, do not stem from any retroperitoneal organs. They can grow to substantial sizes before manifesting symptoms and are typically incidentally detected. According to the urogenital theory, these neoplasms are thought to arise from residual embryonic urogenital system tissues, involving both epithelial and mesothelial components. While vague abdominal pain and distension manifest in fifty percent of cases, clinical manifestations of retroperitoneal cysts are absent in an equivalent proportion of patients, often leading to incidental detection. Acute abdominal pain may arise in some instances if hemorrhage or infection occurs.

Case Reports

Case 1:

A 63 years old female presented with diffuse abdominal pain and distension of abdomen for a long period. Her bowel habits were normal, and she had no urgency, hesitancy, weak stream, or burning sensation when urinating. There was no fever, body weight loss, or recent history of trauma, surgery and chronic pancreatitis, or other diseases of the pancreas. USG of that patient suggestive of large cystic mass in left side of abdomen with low levels of internal echoes. Basic haematological investigations appears unremarkable. S, Amylase and S.Lipase are within normal limits. Serum AFP, B-HCG, CA-125 and CEA levels are within normal limit. Then patient underwent CT scan of Abdomen and Pelvis with I.V. contrast.

CT scan post contrast axial , coronal and sagittal reformatted images of lumbar region showing well defined lobulated fluid density lesion (blue arrows ) with few thin internal septations (green arrow). Note Urinary bladder (red arrow) is well separated from the lesion.

Specimen Biopsy:

 Specimen biopsy shows Fibrocollagenous (blue arrow) cyst wall shows scattered lymphoid aggregates ( green arrow ) with congested blood vessels. No e/o true epithelial lining is visible in cyst wall. Luminal surface of cyst wall (Red arrow). Fibrocollagenous cyst wall shows scattered lymphoid aggregates with congested blood vessels.Few areas shows foreign body giant cell reaction with cholesterol clefts, hemosiderin laden macrophages.True epithelial lining is not evident.No definite granuloma, parasite or malignancy seen.Finding s/o Benign cyst - pseudocyst of retroperitoneum.

Case 2:

Presence of well defined lobulated fluid density lesion is noted involving retroperitoneal compartment in pelvic region. Lesion shows few thin internal septations. Lesion displaces all the pelvic vessels with preserved fat plane. Lesion does not show direct contact with the pancreas, psoas muscle, both kidneys. P/o Cystic lesion arising from retroperitoneum appears likely. Rest of the abdomen and pelvis appears unremarkable.

CT scan post contrast axial , coronal and sagittal reformatted images of pelvic region showing well defined lobulated fluid density lesion (Blue arrows ) with few thin internal septations. Note Urinary bladder (red arrow) is well separated from the lesion. Specimen biopsy Cyst wall composed of fibrous tissue containing chronic lymphoplasmacytic cell infiltration and few haemosiderin laden macrophages. No definite cyst wall lining epithelium identified. No definite granuloma, parasite or malignancy seen. Findings s/o Retroperitoneal pseudocyst – non pancreatic origin likely.

Specimen biopsy

Cyst wall composed of fibrous tissue (blue arrow) containing chronic lymphoplasmacytic cell infiltration(red arrow). Cyst lumen is demonstrated with black arrow. Imaging differentials of the retroperitoneal cystic mass lesion : 1. Neuro-enteric cyst :- May shows communication with the spinal canal, usually occurs in thoracic region. 2. Anterior sacral meningocele :- Usually connected with the sacral vertebrae and may shows the direct communication with the spinal canal foramina. 3. Retroperitoneal lymphatic malformation :- well-defined multilocular cystic masses with typically homogeneous fluid content. Calcifications are uncommon. Post-contrast enhancement of the cyst wall and septa may be seen. they can cross fascial planes and involve multiple compartments 4. Retroperitoneal hydatid cyst :- As hydatid cyst can also involves retroperitoneal compartment of the abdomen. 5. Pancreatic pseudocyst :- common sequelae of acute pancreatitis or chronic pancreatitis, and the most common cystic lesion of the pancreas.

A retroperitoneal non-pancreatic pseudocyst is an uncommon surgical condition that presents a variety of differential diagnoses, with a reported incidence ranging from 1 in 5750 to 1 in 250,000 (3). The retroperitoneal pseudocysts typically remain asymptomatic until they attain a considerable size and begin to exert pressure on neighboring structures, often being incidentally identified [6]. Two main theories exist regarding the development of such an enlarged cyst within the retroperitoneal region: 1. Within the retroperitoneal space lie organs that have their origins in both the ectoderm and endoderm, enclosed by a lax connective tissue framework. In this scenario, both primary and metastatic tumors may proliferate silently, manifesting symptoms only upon reaching a significant size. Cysts stemming from the Wolfan duct exhibit a distinct fluid content, while teratomatous cysts are characterized by the presence of sebaceos material [4]. 2. Due to the absence of a mesothelial lining in the retroperitoneal space, the surplus fluid cannot be reabsorbed, leading to the considerable expansion of pseudocysts [6]. Imaging helps in deciding the exact localisation and extent of the lesion and guide the surgeon for the appropriate approach. CT scan is the modality of the choice which delineates the appropriate relation of the structures with the cystic lesion. The definitive treatment for retroperitoneal cysts involves complete excision. Marsupialization and partial excision are discouraged due to the high likelihood of recurrence.

Primary non-pancreatic retroperitoneal pseudocysts are very rare lesions and have to be distinguished from other differential diagnoses of retroperitoneal lesions, and a surgeon should be aware of the possible occurrence of these lesions with unknown origin. Imaging plays a crucial role in localisation and exact extent of the lesion as well as relationship with the surrounding structures. Surgical excision is the only way to exclude malignancy and confirm the diagnosis.

1. Cofn A, Boulay-Coletta I, Sebbag-Sfez D, Zins M. Radioanatomy of the retroperitoneal space. Diagn Interv Imaging. 2015;96(2):171–86. https:// doi.org/10.1016/j.diii.2014.06.015.
2. Scali EP, Chandler TM, Hefernan EJ, Coyle J, Harris AC, Chang SD. Primary retroperitoneal masses: what is the diferential diagnosis? Abdom Imaging. 2015;40(6):1887–903. https://doi.org/10.1007/s00261-014-0311-x.
3. Latif E, Musthafa S, Ahmed A, Khanna M. Idiopathic retroperitoneal nonpancreatic pseudocyst in an adult male: radiological images and surgical video of laparoscopic excision. Cureus. 2020;12(3): e7243. https://doi.org/ 10.7759/cureus.7243.
4. Palanivelu C, Rangarajan M, Senthilkumar R, Madhankumar MV, Annapoorni S. Laparoscopic excision of an infected “egg-shelled” retroperitoneal pseudocyst. J Gastrointestin Liver Dis. 2008;17(4):465–8.
5. Geng J-H, Huang C-H, Wen-Jeng Wu, Huang S-P, Chen Y-T. Huge retroperitoneal nonpancreatic pseudocyst. Urol Sci. 2012;23(2):61–3. https://doi. org/10.1016/j.urols.2012.03.002.
6. Durczyński A, Hogendorf P, Szymański D, Strzelczyk J. First report worldwide of huge retroperitoneal pseudocyst with high fluid concentration of CA 125. Med Sci Tech. 2011;52(1–2):67–9.