What Is Retroperitoneal Fibrosis?

Retroperitoneal fibrosis is a rare condition in which a mass of fibrous, inflamed tissue grows in the retroperitoneum, the space behind the abdominal cavity that houses the aorta, kidneys, and ureters. The tissue typically wraps around the abdominal aorta and iliac arteries and then spreads outward, often encasing the ureters and blocking urine flow from the kidneys.¹ Because the symptoms are vague and the disease is uncommon, it can go unrecognized for months, sometimes turning up only after kidney function has already deteriorated.

How Common It Is and Who It Affects

A prospective study in the Netherlands calculated an annual incidence of about 1.3 cases per 100,000 people, with a mean age at diagnosis around 64 and a male-to-female ratio of roughly 3 to 1.² Men in their fifties through seventies make up the bulk of cases, which partly explains why the condition is rarely suspected in younger patients. When it does appear in a younger person, it tends to catch clinicians off guard, since nonspecific abdominal pain and early signs of urinary obstruction have a long list of more common explanations.³

What Causes It

About 70 percent of retroperitoneal fibrosis cases are classified as idiopathic, meaning no single external trigger can be identified.⁴ The remaining 30 percent are considered secondary, arising from medications, radiation therapy, malignancy, infections, or prior abdominal surgery. But the line between “idiopathic” and “explained” has blurred considerably in recent years, as researchers have uncovered immune-driven mechanisms behind many supposedly unexplained cases.

The Immune Theory and IgG4-Related Disease

One leading explanation involves an autoimmune-like reaction centered on the aorta. In people with atherosclerotic plaques, oxidized lipids that are normally walled off inside the artery can leak through damaged vessel walls. The immune system recognizes these leaked molecules as foreign, mounting an inflammatory attack that produces the dense fibrous tissue characteristic of the disease.⁵ This theory helps explain why the fibrosis almost always clusters around the aorta and why similar fibrous masses occasionally show up around the thoracic aorta in the chest.

A significant subset of idiopathic cases turns out to be part of IgG4-related disease, a systemic condition in which a specific type of immune cell infiltrates and scars various organs. Biopsies of the retroperitoneal mass in these patients show dense clusters of IgG4-positive plasma cells.⁶ One study of biopsy-confirmed cases found that 57 percent qualified as IgG4-related, with distinctive histological features including a swirling pattern of scar tissue and prominent immune-cell infiltration.⁷ Recognizing the IgG4 subtype matters because these patients may have fibrosis or inflammation in other organs at the same time, and the distinction can influence treatment choices.

Drug-Induced Cases

Medications are the best-known secondary cause. Historically, the biggest culprits were ergot alkaloid derivatives such as ergotamine and methysergide, which were widely prescribed for migraines and Parkinson’s disease. Long-term use triggered fibrosis through a mechanism thought to involve serotonin-receptor stimulation.⁸ As those drugs have been phased out or restricted in many countries, drug-induced retroperitoneal fibrosis has become less common from that source, but newer medications have taken their place. Reports now link the condition to certain chemotherapy agents, biologic drugs, and a surprisingly broad range of commonly prescribed medications.⁹

A large French pharmacovigilance study found that while ergot derivatives still had the strongest statistical association with retroperitoneal fibrosis, significant links also appeared with beta-blockers, statins, antiplatelet drugs, antiepileptic medications, antidepressants, and TNF-alpha antagonists used for autoimmune conditions.¹⁰ The absolute risk from any individual drug remains very low, but the breadth of the list is a reminder that drug-induced cases can be easy to miss.

Asbestos and Smoking

Two environmental exposures stand out. A case-control study found a strong association between occupational asbestos exposure and retroperitoneal fibrosis, with the odds of developing the disease roughly four to nine times higher depending on the degree of exposure.¹¹ Smoking for more than 20 pack-years was also an independent risk factor in that study, and later research confirmed the link, finding that both current and former smokers were about three times more likely to develop the condition than people who had never smoked. The combination of smoking and asbestos exposure was particularly hazardous, with an odds ratio above 12.¹² Smoking likely feeds the atherosclerotic-plaque pathway described earlier, while asbestos may provoke chronic inflammation in retroperitoneal tissues directly.

Cancer and Retroperitoneal Fibrosis

Cancer can either mimic retroperitoneal fibrosis or cause a truly fibrotic reaction. A study examining cancer risk among retroperitoneal fibrosis patients found that the rate of cancer diagnoses within one year of the fibrosis diagnosis was roughly ten times higher than in the general population. The cancers found in that window tended to involve organs in the retroperitoneum itself, including the kidney, renal pelvis, and pancreas, and they were more likely to be at an advanced stage.¹³ Beyond that first year, the cancer rate returned to normal, suggesting that many of these were cancers masquerading as or triggering fibrosis rather than a long-term cancer risk from the disease itself. Because malignant tissue infiltrating the retroperitoneum can look very similar to benign fibrosis on imaging, biopsy is often the only way to tell the two apart.¹⁴

Symptoms

The hallmark symptom is a dull, persistent pain in the lower back, flanks, or abdomen. It tends to build gradually and is easy to mistake for a musculoskeletal problem or kidney stones. As the fibrous tissue encases the ureters, urine backs up into the kidneys, causing hydronephrosis. At that point, patients may notice decreased urine output, swelling, or fatigue from worsening kidney function. Some people present with unexplained kidney failure before anyone suspects the real cause.¹⁵ Flank pain is the most common complaint across both IgG4-related and non-IgG4 forms of the disease.¹⁶

Less commonly, the fibrotic mass can compress blood vessels. When the inferior vena cava is affected, the result may be severe leg swelling from impaired blood return.¹⁷ General symptoms like weight loss, low-grade fever, and elevated inflammatory markers are common but nonspecific, contributing to the diagnostic delay many patients experience.

How It Is Diagnosed

CT scans are typically the first tool to spot the mass, which shows up as a soft-tissue plaque encasing the aorta and surrounding structures. On MRI, untreated disease tends to light up brightly on fluid-sensitive sequences and show strong contrast enhancement, both signs of active inflammation rather than burnt-out scar. PET scans using a glucose tracer can help distinguish active fibrosis from inactive, treated disease: untreated patients showed significantly higher tracer uptake than those who had already received therapy.¹⁸ This distinction is useful for monitoring treatment response and detecting relapses before they cause kidney damage.

Blood tests alone cannot confirm the diagnosis, but they help characterize it. Most patients have elevated inflammatory markers. In the IgG4-related subtype, serum IgG4 levels, eosinophil counts, and IgE levels tend to be higher, and complement levels tend to be lower than in non-IgG4 cases.¹⁹ Patients with elevated serum IgG4 also tend to have higher inflammatory markers and are more likely to have swollen lymph nodes near the mass.²⁰

Despite advances in imaging and blood work, tissue biopsy remains the gold standard. Without it, there is no reliable way to rule out malignancy or to confirm whether the fibrosis is IgG4-related. Laparoscopic or open biopsy provides the most tissue, though CT-guided needle biopsy is a less invasive alternative.²¹

Medical Treatment

Steroids are the first-line therapy for idiopathic retroperitoneal fibrosis. Glucocorticoids, sometimes combined with an additional immunosuppressant, push the disease into remission in roughly 80 to 90 percent of patients.²² In one well-described cohort, steroid-based therapy resolved systemic symptoms in all patients, shrank the periaortic mass by at least 25 percent in nearly nine out of ten, and cleared ureteral obstruction in the vast majority of affected kidneys.²³ The catch is that steroids carry well-known long-term side effects, which has driven interest in alternatives.

Tamoxifen, a drug better known for its use in breast cancer, has antifibrotic properties and has been used as a steroid-sparing option. A cohort study found that most patients on tamoxifen reported symptom relief within a few weeks, and CT scans showed mass shrinkage in about 85 percent after eight months.²⁴ However, when the two drugs were compared head-to-head in a randomized trial, prednisone was clearly better at preventing relapses. By about two years, the estimated relapse rate was roughly 17 percent with prednisone versus 50 percent with tamoxifen.²⁵ Tamoxifen remains a reasonable choice for patients who cannot tolerate steroids, but it is not an equivalent substitute.²⁶

For patients whose disease does not respond to steroids or who relapse repeatedly, stronger immunosuppressants come into play. Options include oral agents such as methotrexate, mycophenolate mofetil, and azathioprine, as well as intravenous therapies like cyclophosphamide and rituximab.²⁷ Rituximab, which targets B cells, has shown promise in steroid-refractory cases with high inflammatory activity on PET scans.²⁸ In one case report, a combination of rituximab, cyclophosphamide, and dexamethasone achieved complete disappearance of the fibrotic mass on PET imaging within four months, with sustained remission at three years on maintenance rituximab.²⁹

Ureteral Drainage and Surgery

When the ureters are blocked and kidney function is threatened, restoring urine flow is an immediate priority. The two standard approaches are internal ureteral stents, threaded up through the bladder, and external percutaneous nephrostomy tubes, placed through the back directly into the kidney. A study tracking outcomes found that initial stent placement succeeded in about 79 percent of attempts, and most of those patients could be managed with stents alone. Percutaneous nephrostomy served as a reliable backup. Complication rates were similar between the two methods.³⁰

These drainage procedures are usually temporary bridges while medical therapy takes effect. When obstruction persists despite medication, or when the diagnosis needs to be confirmed surgically, ureterolysis is the definitive procedure. The surgeon frees the ureters from the encasing fibrous tissue and then wraps them in omentum, a fatty apron of tissue from the abdomen, or repositions them inside the peritoneal cavity to prevent re-encasement. Success rates for this approach exceed 90 percent historically.³¹ The procedure is increasingly done laparoscopically or robotically, offering a less invasive recovery than traditional open surgery while maintaining high success rates.³² For fibrosis caused specifically by pelvic radiation, ureterolysis combined with omental wrapping also appears effective.³³

Relapse and Long-Term Outlook

Getting the disease into remission is the straightforward part. Keeping it there is the challenge. Across multiple long-term studies, roughly a third to half of patients relapse at least once, often years after the initial response. One cohort study found cumulative relapse rates of about 21 percent at five years and 48 percent at fifteen years.³⁴ Another reported even steeper numbers, with relapse reaching about 62 percent by seven and a half years, though those patients had longer follow-up.³⁵

Several factors help predict who will relapse. Patients who presented with hydronephrosis at diagnosis carried a substantially higher risk. Positive antinuclear antibodies, male sex, onset with acute kidney injury, smoking, and low back pain at presentation have all been identified as relapse predictors across different studies.³⁶ On the protective side, continuing maintenance glucocorticoid therapy significantly reduced relapse risk. Stopping maintenance therapy prematurely roughly tripled the hazard of relapse in one cohort.³⁷ This creates a real clinical dilemma: the disease often demands years of low-dose steroid or immunosuppressive therapy, and patients have to weigh the side effects of ongoing treatment against the risk of flare-ups that can silently damage the kidneys.

Diagnostic Mimics and Overlooked Triggers

One of the trickiest aspects of retroperitoneal fibrosis is that it can masquerade as other conditions and vice versa. The vague pain, elevated inflammatory markers, and imaging findings can be confused with lymphoma, sarcoma, or metastatic cancer in the retroperitoneum. Even within confirmed fibrosis, distinguishing benign from malignant takes tissue: imaging alone can miss tumor cells embedded in the scar. That is why experienced centers push for biopsy even when the CT appearance looks “classic.”

On the flip side, conditions like deep endometriosis in the pelvis can produce imaging that resembles retroperitoneal fibrosis, leading to misdiagnosis. In at least one documented case, a young woman was diagnosed with retroperitoneal fibrosis and treated with steroids and tamoxifen without improvement before pelvic endometriosis was identified as the actual source of her symptoms and ureteral obstruction. The overlap is rare, but it highlights the risk of anchoring on the fibrosis diagnosis without considering gynecologic causes in premenopausal women.

Drug-induced cases also deserve more suspicion than they typically get. Because so many common medications have been statistically linked to the condition, clinicians do not always make the connection. Beta-blockers and statins are prescribed to hundreds of millions of people worldwide; the fibrosis risk from any individual prescription is vanishingly small, but at the population level it is not zero. If you have been diagnosed with retroperitoneal fibrosis, a thorough medication review is a reasonable part of the workup, even if no single drug on your list is a “known” offender.

When the Disease Involves More Than the Retroperitoneum

IgG4-related retroperitoneal fibrosis does not always stay confined to one area. Because IgG4-related disease is systemic, patients can develop fibrotic or inflammatory masses in the salivary glands, pancreas, bile ducts, orbits, lungs, or thyroid. If you are diagnosed with retroperitoneal fibrosis and also have unexplained swelling in the face, pancreatic inflammation without heavy alcohol use, or other organ involvement that does not quite fit any single diagnosis, IgG4-related disease should be high on the list. Serum IgG4 testing and biopsy of accessible affected tissue can confirm the connection.³⁸ The distinction matters for treatment: these patients may benefit from rituximab or other targeted therapies and need monitoring of all involved organs, not just the retroperitoneum.

The occasional link between retroperitoneal fibrosis and mediastinal fibrosis, a similar condition involving the chest, also fits the IgG4 and autoimmune frameworks. If the underlying problem is a systemic immune reaction to oxidized lipids leaking from diseased arteries, it makes sense that the process could develop wherever large arteries are present.³⁹ Patients diagnosed with fibrosis in one location are sometimes screened for involvement in others, though no formal guidelines exist on how aggressively to look.