IMAGING

PLAIN X-RAY

Conventional radiographs have no imaging role in the diagnosis of RPF [7].

ABDOMINAL X-RAY

Inespecific. In advanced stages, a central mass of soft tissue at the level of the psoas line is sometimes seen.

Fig 5: Plain abdominal x-ray shows blurring of the right psoas fat line (arrowheads).

ULTRASONOGRAPHY (US)

Useful as screening, but with low sensitivity.

RPF is visualized as a hypo or anechoic, irregular demarcated retroperitoneal mass, anterior to the lower lumbar spine or the promontory of the sacrum.

Fig 6: US findings helping to determine the origin of retroperitoneal fibrosis.

If ureteral entrapment is present, ureterohydronephrosis may occur in varying degrees.

MULTIDETECTOR COMPUTED TOMOGRAPHY (MDCT)

Useful to assess other conditions associated with RPF (IgG4 related diseases such as autoimmune pancreatitis or biliary structures ductal dilation).

MDCT may fail to demonstrate abnormalities in approximately one third of surgically proven RPF cases.

RPF typically appears as a well-defined, irregular paraspinal soft-tissue mass, isodense to the psoas muscle and without lateral extension beyond it. Most often centred at L4–L5 near the aortic bifurcation, it may extend cranially towards the renal hila or, less commonly, caudally to involve pelvic structures. Baseline Hounsfield units (HU) values and the diameter of the retroperitoneal tissue can assist in assessing inflammatory activity. [1,3,5-8]

Fig 7: CT imaging findings that help to determine the activity of RPF and its response to treatment.

Fig 8: How to determine the origin of retroperitoneal fibrosis on CT imaging.

MAGNETIC RESONANCE IMAGING (MRI)

MRI offers superior soft-tissue contrast and enables non-contrast visualisation of the renal collecting system, making it particularly valuable in patients with renal impairment.

RPF is typically hypointense on T1-weighted images with variable intensity on T2-weighted images as well as in apparent diffusion coefficient values depending on the degree of active inflammation [2,3].

Fig 9: MRI retroperitoneal fibrosis imaging findings according to the phase of activity.

NUCLEAR IMAGING

FDG-PET/CT enables reliable assessment of metabolic activity in idiopathic retroperitoneal fibrosis, supporting diagnosis, treatment monitoring and prognostic stratification. Increased FDG avidity is associated with active disease and may predict response to immunosuppressive therapy, while metabolically inactive tissue can guide ureteric stent removal. Although CT and MRI remain the preferred modalities for routine follow-up, FDG-PET/CT is valuable when residual disease activity is uncertain. Higher SUVmax and contrast-enhancement scores correlate with inflammatory markers, with SUVmax > 2.76 accurately identifying active disease.

BIOPSY

Biopsy is indicated when RPF is suspected on clinical, laboratory, or imaging findings, and is particularly crucial in cases with atypical anatomical distribution or inadequate therapeutic response. Histopathological evaluation enables distinction between IgG4-related and non-IgG4-related variants of the disease.

Fig 10: Proposed diagnostic algorithm.

CASES

Fig 11: CASE 1. Idiopathic retroperitoneal fibrosis. 67-year-old male with dysuria and pollakiuria of 1 week's duration. Ultrasound reveals obstruction of the left ureter with grade II/IV ureterohydronephrosis (A) secondary to a solid midline mass (B). CT without contrast (C) and with intravenous contrast (D) shows this retroperitoneal mass extending periaortically, without contrast uptake.

Fig 12: CASE 1. MRI shows thickening of the periaortic soft tissues, without evident enhancement, corresponding to retroperitoneal fibrosis.

Fig 13: CASE 2. Idiopathic retroperitoneal fibrosis. Retroperitoneal solid mass of soft tissue density (orange circle) corresponding with RPF, encompassing the aorta and inferior vena cava and causing bilateral ureteral obstruction, treated with double-J stents (green arrows).

Fig 14: CASE 3. Mycotic aortic aneurysm. Saccular dilation of the right posterior part of the abdominal aorta at the level of the celiac trunk. Associate perilesional soft tissue enlargement. PET-CT shows increased annular metabolism at the location of the periaortic soft tissue mass identified on the CT scan, with a SUVmax 8.10, which increases to 11 on the later study, consistent with an active infectious process.

Fig 15: CASE 4. Retroperitoneal fibrosis of probable inflammatory origin secondary to aortic involvement. Male patient with a prothesis in the descending aorta due to aneurysmal dilation. At the first post-procedure follow-up (A), increased soft tissue density was visualized in the renal sinuses and proximal ureters, which did not enhance after contrast administration. At the one-year follow-up (B), partial improvement was observed. However, two years after the surgical procedure (C), an increase in soft tissue density was noted. D,E: no systemic involvement was observed, nor was thoracic or skeletal involvement.

Table 1: Summary of retroperitoneal fibrosis findings among different imaging modalities.

DIFFERENTIAL DIAGNOSES

RETROPERITONEAL LYMPHOMA

Lymphoma is the most common malignant retroperitoneal lesion, being either a Hodgkin lymphoma or non-Hodgkin lymphoma.

CT typically shows a homogeneous, well-defined, bulky mass displacing anteriorly but not compressing adjacent retroperitoneal structures. Superior extension above L4–L5 is more frequently observed. Post-contrast CT usually demonstrates homogeneous enhancement, although peripheral or heterogeneous enhancement may occur in the presence of necrosis. 

Fig 16: Retroperitoneal lymphoma. Soft tissue mass, periaortocaval without compression of its lumen and with homogeneous enhancement (orange). Anterior displacement of the iliac arteries: floating vessel sign (arrow).

On MRI tends to be isointense on T1-weighted imaging and iso- to hypointense on T2-weighted sequences, with moderate and often heterogeneous enhancement after contrast administration.

Although enhancement may be present in both malignant and idiopathic retroperitoneal fibrosis, a hypointense T2 signal is far more suggestive of chronic or inactive idiopathic RPF. Differentiation on FDG-PET/CT can also be challenging; however, malignant aetiologies typically demonstrate higher FDG avidity and greater SUVmax values. In comparative studies, lymphoma and metastatic disease showed substantially higher metabolic activity than idiopathic RPF, and were more frequently associated with lymphadenopathy at distant nodal stations [3].

Fig 17: Retroperitoneal lymphoma. Extensive soft tissue density in the interaortocaval and periaortic regions (green) extending further to the left side, affecting the renal hilum and perinephric space (arrow).

ERDHEIM-CHESTER DISEASE (ECD)

ECD is a rare non-Langerhans cell histiocytosis affecting multiple organ systems, typically presenting in adults in their 5th–7th decades with slight male predominance.

Diagnosis relies on biopsy supported by characteristic clinical and imaging features.

Retroperitoneal and renal involvement occurs in a significant proportion, with CT and MRI demonstrating bilateral perinephric and posterior pararenal soft-tissue infiltration, producing the classic “hairy kidney” appearance. Infiltrates are generally isoattenuating or iso- to hypointense to skeletal muscle with mild contrast enhancement; FDG uptake may be present but can be obscured by physiological renal activity. Chronic infiltration may result in renal atrophy.

Fig 18: Bilateral perinephric and posterior pararenal soft-tissue infiltration, producing the “hairy kidney” appearance.

Although both ECD and retroperitoneal fibrosis can lead to hydronephrosis, their distribution differs: ECD typically affects the perinephric fat and renal hila, whereas idiopathic RPF encases the anterolateral aorta and proximal ureters. Aortic encasement (“floating aorta sign”) and periureteric involvement may occur, but ECD usually spares the IVC and pelvic ureters. Additional distinguishing features include irregular perirenal infiltration and bilateral symmetric adrenal thickening.

Fig 19: Erdheim-Chester retroperitoneal disease. Increased echogenicity of the surrounding tissue (asterisks) in both the right (A) and left (B) kidneys. Abdominal CT with intravenous contrast (C, D) shows bilateral thickening of the perinephric soft tissue density, producing the appearance of "hairy" kidneys.

Skeletal involvement, particularly of the femur, tibia and fibula, is seen in most patients, often causing bone pain [9].

Fig 20: Erdheim-Chester retroperitoneal disease. A. Periureteral and right periaortic soft tissue density thickening (arrow). B, C. Concentric and bilateral perirenal soft tissue density thickening with contrast enhancement, giving a "hairy" kidney appearance (arrow) and periureteral soft tissue density mass (arrowhead).

Fig 21: Erdheim-Chester retroperitoneal disease. Bilateral and symmetrical generalized infiltration by soft tissue surrounding both kidneys and proximal ureters. (orange). It is hypointense on T1(B) and T2-weighted sequences (A, C, D).

Fig 22: Erdheim-Chester retroperitoneal disease. Multiple bilateral metaphyseal and diaphyseal sclerotic lesions (arrows) with FDG uptake with SUVmax 3.8 on FDG F18 PET-CT.

EXTRA-MEDULLARY HEMATOPOIESIS (EMH)

EMH is often incidental and represents compensatory blood cell production outside the bone marrow as a compensatory mechanism for [10]:

• Decreased bone marrow hematopoiesis or increased peripheral destruction of blood cells.
• Secondary to bone marrow infiltration.

Although most cases present microscopically with homogeneous hepatosplenomegaly, macroscopic EMH may appear at several sites, including the thorax (most commonly as paraspinal masses), liver and spleen, and retroperitoneum, where it commonly appear as unilateral or bilateral soft-tissue masses, frequently involving the perinephric space [11]. These lesions typically form a soft-tissue rind without renal mass effect, with foci of fat-attenuation may be interspersed within the regions of soft tissue, and are hypovascular on post-contrast imaging. Calcifications are uncommon.

The characteristic perinephric distribution generally distinguishes EMH from retroperitoneal fibrosis, though differentiation becomes difficult when RPF exhibits atypical peri-renal extension. Supporting features such as hepatosplenomegaly or skeletal abnormalities related to the underlying haematological disorder (e.g., sclerotic bone in myelofibrosis or “H-shaped” vertebrae in sickle-cell disease) aid in establishing the correct diagnosis.

Depending on the phase, we will observe different radiological characteristics:

Table 2: Radiological characteristics of Extra-medullary hematopoiesis.

Fig 23: Extramedullary hematopoiesis foci. Hyperintense thoracoabdominal paravertebral masses on T2-weighted sequences (orange arrows) in relation to extramedullary hematopoiesis foci in the acute phase, and hypointense thoracic masses (white arrow) due to chronic hematopoiesis foci. Diffuse bone expansion of posterior rib arches (green arrows).

RETROPERITONEAL METASTASIS AND ADENOPATHIES

Some tumors, such as prostate, cervical, breast, and lung cancer, can present with retroperitoneal metastases that manifest as:

• A cuff of soft tissue along with a desmoplastic response, similar to retroperitoneal fibrosis.
• Usually more focal or asymmetric compared to retroperitoneal fibrosis.
• Signal intensity on T2-weighted sequences is higher and more heterogeneous.
• Additional lymphadenopathy also present in the pelvis and retroperitoneal region.

Fig 24: Metastatic lymph node conglomerate. Periaortic mass, soft tissue density, related to metastatic lymph node conglomerate (orange). Biliary prosthesis (blue arrow).

TUBERCULOSIS

The most common focus of extra-pulmonary tuberculosis is the abdomen. Abdominal tuberculosis frequently involves liver, spleen, genitourinary tract and retroperitoneal lymph nodes and retroperitoneal abscess as extension from adjacent spinal disease [3, 12, 13].

• Retroperitoneal lymphadenopathy often appear with central hypoattenuation and peripheral rim enhancement on CT, suggestive of caseous necrosis.
• Hepatosplenic involvement may be micronodular or macronodular, with CT and MRI appearances overlapping metastases or abscesses; calcified granulomas can aid diagnosis.
• Genitourinary tuberculosis commonly affects the kidneys, ureters, bladder, and prostate, usually unilaterally. Renal findings include parenchymal oedema and hypoperfusion manifesting on CT as geographic areas of parenchymal hypoattenuation that mimic acute pyelonephritis and abscess (acute tuberculous renal abscess are 10-40 HU with mild peripheral enhancement) in the active inflammation, calyceal erosions, papillary necrosis, parenchymal cavitation, and calcifications in the chronic phase, potentially progressing to autonephrectomy. Ureteric involvement presents as wall thickening and strictures, predominantly in distal segments, leading to hydroureteronephrosis.

Fig 25: Tuberculosis. Periaotic mass with heterogeneous density associated with necrotic lymph node conglomeration (arrowhead). Splenic (white arrow) and renal (orange arrow) abscesses due to hematogenous spread.

Table 3: Summary table of the main differential diagnoses.