Epidemiological Profile

• Incidence: 1 per 200,000 to 1 per 500,000 person-years (varies by geographic region)
• Gender Predominance: Males 2-3:1 (unexplained; no clear hormonal mechanism identified)
• Age of Onset: Peak incidence 50-60 years; rare in childhood and young adults
• Anatomical Involvement: Bilateral ureteral obstruction in majority of cases (~75%); may extend cranially along aorta or caudally along iliac vessels

The relative rarity of RPF and its nonspecific presentation often lead to diagnostic delays of 6-24 months from symptom onset to diagnosis. In contemporary practice, approximately 70% of idiopathic RPF cases are now recognized as IgG4-related disease (IgG4-RD), fundamentally changing our understanding of its etiology and treatment strategy.

Idiopathic RPF (Approximately 70% of Cases)

Historical Context: Idiopathic RPF was long considered a primary inflammatory condition with unknown etiology. The 2011 discovery of IgG4+ plasma cell infiltration in retroperitoneal tissue—coupled with elevated serum IgG4 levels and responsiveness to corticosteroids—led to reclassification of most idiopathic cases as IgG4-related disease.

Pathophysiology: - Autoimmune/inflammatory pathogenesis driven by aberrant T-cell and B-cell responses - IgG4+ plasma cell infiltration with storiform fibrosis and lymphoplasmacytic inflammation - TGF-β activation promoting fibroblast proliferation and extracellular matrix deposition - Potential role of molecular mimicry (antigen cross-reactivity with environmental triggers)

Association with Other Fibrosing Conditions: Patients with idiopathic (IgG4-related) RPF have increased prevalence of concurrent or sequential fibrosing diseases affecting other organ systems: - Mediastinal fibrosis (anterior or posterior mediastinal mass) - Orbital pseudotumor (IgG4-related orbital disease) - Autoimmune pancreatitis (focal or diffuse pancreatic inflammation) - Sclerosing cholangitis (IgG4-associated biliary strictures; distinct from primary sclerosing cholangitis) - Riedel’s thyroiditis (fibrous infiltration of thyroid) - Systemic sclerosis-like features (skin involvement, fibrosis)

These associations warrant screening imaging (chest CT, abdominal ultrasound, endoscopic ultrasound if pancreatitis suspected) in newly diagnosed RPF patients.

Secondary RPF (Approximately 30% of Cases)

Secondary RPF arises from identifiable external causes and does NOT typically involve IgG4+ plasma cell infiltration. Treatment differs fundamentally from idiopathic disease.

Malignancy-Associated RPF: - Lymphoma (Hodgkin and non-Hodgkin; most common malignant association) - Sarcomas (fibrosarcoma, liposarcoma, leiomyosarcoma) - Metastatic carcinoma: breast, colon, prostate, gastric, pancreatic - Distinction from idiopathic RPF: imaging shows invasion with destruction of adjacent structures; tissue diagnosis reveals malignant cells; no response to steroids

Drug-Induced RPF: - Ergot Alkaloid Derivatives (now rare in developed countries): - Methysergide (migraine prophylaxis; withdrawn in US 1983) - Ergotamine (acute migraine; risk proportional to cumulative dose and duration) - Mechanism: altered serotonergic signaling promoting fibroblast proliferation - Beta-Blockers: - Practolol (withdrawn; associated with severe oculomucocutaneous syndrome with RPF) - Modern beta-blockers (metoprolol, atenolol, propranolol) at standard doses: negligible risk - Other Agents (rare): - Hydralazine - Methyldopa - Cisapride (prokinetic agent; withdrawn from US market in 2000 due to QT prolongation; associated with rare RPF cases prior to withdrawal)

Infection-Associated RPF: - Tuberculosis (most common infection-related cause; granulomatous inflammation) - Actinomycosis (chronic suppurative infection; fungal-like bacteria) - Histoplasmosis (endemic fungi; granulomatous fibrosis) - Diagnosis requires culture, tissue demonstration of organisms, or molecular testing

Radiation-Induced RPF: - Occurs 5-20 years after abdominal or pelvic radiation therapy - Mechanism: endothelial injury, hypovascular ischemia, TGF-β activation - Associated with lymphocyte infiltration (not IgG4-predominant) - Risk increases with total dose >50 Gy and dose fractionation

Post-Surgical RPF: - Follows aortic aneurysm repair (both open and endovascular) - Occurs in ~5% of AAA repair patients; mechanisms include foreign body reaction to graft and ischemic injury - Occurs 1-10 years post-operatively - Management: conservative observation if stable; surgical intervention only if progressive obstruction

Asbestos Exposure: - Long latency period (20-50 years) - Associated with pleural plaques, pulmonary fibrosis, or mesothelioma history - Nonspecific inflammatory pathology; not IgG4-associated

Overview of IgG4-Related Disease

IgG4-RD is a systemic fibroinflammatory condition characterized by dense lymphoplasmacytic infiltration rich in IgG4+ plasma cells, storiform fibrosis, and obliterative phlebitis affecting multiple organ systems. RPF is now recognized as a cardinal manifestation of IgG4-RD, accounting for approximately 10% of patients with systemic IgG4-RD and being present in approximately 40-70% of patients with idiopathic RPF.

2020 American College of Rheumatology / European League Against Rheumatism (ACR/EULAR) Classification Criteria for IgG4-RD: - Require: elevated serum IgG4 levels AND histopathologic evidence of IgG4+ plasma cell infiltration - Involved organs: RPF, autoimmune pancreatitis, sclerosing cholangitis, lacrimal gland enlargement, salivary gland enlargement, thyroiditis, and others - Classification-level sensitivity and specificity >90% when both serologic and histopathologic criteria met

Serum IgG4 Levels in RPF

Critical Caveat: Elevated serum IgG4 is neither necessary nor sufficient for diagnosis of IgG4-RD or IgG4-related RPF.

• Elevated serum IgG4 (>135 mg/dL) present in approximately 40% of idiopathic RPF patients
• Approximately 30-40% of patients with confirmed IgG4-RD (by tissue criteria) have normal serum IgG4 levels
• Up to 5-10% of apparently healthy subjects have mildly elevated IgG4 without disease manifestations
• Therefore: Normal serum IgG4 does NOT exclude IgG4-RD; tissue diagnosis is gold standard

Histopathological Diagnosis of IgG4-RD

When tissue is obtained (by CT-guided biopsy, laparoscopy, or open biopsy), examine for all four features:

1. Storiform Fibrosis (pathognomonic): Whirled, woven pattern of fibrosis; distinguished from simple linear or keloid-type fibrosis by presence of concentric lamellae
2. Lymphoplasmacytic Infiltrate: Dense infiltration of B lymphocytes and plasma cells, often with prominent germinal centers
3. Obliterative Phlebitis: Inflammation and partial or complete occlusion of small to medium-sized veins; can mimic vasculitis but vessels are NOT necrotic (IgG4-RD is NOT a true vasculitis)
4. IgG4+ Plasma Cell Infiltration: >10 IgG4+ cells per high-power field (HPF) on immunohistochemistry; IgG4+/IgG total plasma cell ratio >40% supports diagnosis

Grading Systems: - Consensus statement (2012): Definite = all 4 features; Probable = 3 of 4; Possible = 2 of 4; Suspicious for Non-IgG4-RD = 0-1 feature - “Responder phenotype”: Patients with high IgG4+ plasma cell infiltration typically show excellent response to corticosteroids; those with predominant fibrosis and lower IgG4+ counts may require additional immunosuppression

Other Organ Manifestations of IgG4-RD

When IgG4-related RPF is diagnosed, screen for involvement of other organ systems:

Pancreatic: - Autoimmune pancreatitis (Type 1, IgG4-associated; Type 2 is distinct and IgG4-negative) - Presents as diffuse or focal pancreatic enlargement (may mimic cancer) - Associated with sclerosing cholangitis in ~75% of Type 1 AIP cases - Risk of pancreatic insufficiency (diabetes, steatorrhea)

Biliary: - IgG4-associated sclerosing cholangitis: segmental or diffuse bile duct stricturing - Distinguished from primary sclerosing cholangitis (PSC) by isolated involvement, normal or elevated IgG4, and response to steroids - Increased risk of cholangiocarcinoma (as with PSC)

Salivary and Lacrimal Glands: - Enlarged submandibular and parotid glands - Lacrimal gland enlargement (orbital pseudotumor) - Distinct from Sjögren’s syndrome (SSA/Ro and SSB/La antibodies are negative in IgG4-RD)

Thyroid: - Riedel’s thyroiditis: fibrous infiltration of thyroid gland with woody firmness; may present with hypothyroidism or thyroid mass effect - Distinguished from Hashimoto’s thyroiditis (no TPO/TSI antibodies in IgG4-RD, though overlap exists)

Aortic: - IgG4-associated aortitis: thoracic or abdominal aortic inflammation and thickening - Risk of aortic aneurysm and dissection - May coexist with RPF in same patient

Renal Parenchymal: - Tubulointerstitial nephritis (TIN) with IgG4+ plasma cell infiltration - Rare as sole manifestation; often accompanies RPF - May contribute to renal dysfunction independent of obstruction

Mechanism of PSA Elevation in RPF

Elevated PSA in the setting of RPF occurs via a distinct mechanism unrelated to malignancy:

1. Bilateral Ureteral Obstruction → Urinary Stasis
2. Elevated Intraprostatic Pressure (backup from obstructed bladder outflow and retrograde pressure transmission)
3. Prostatic Congestion, Edema, and Cellular Damage
4. Leakage of Prostate-Produced PSA into Circulation

PSA is neither cancer-specific nor prostate-tissue-specific; it is present in normal prostate epithelial cells and is released into serum with cellular injury from any etiology.

Distinction from Prostate Cancer

RPF with PSA Elevation: - PSA elevation mild to moderate (typically <30 ng/mL, though higher levels reported) - PSA density (PSA/prostate volume) abnormal - Digital rectal exam: normal or mildly boggy prostate - Transrectal ultrasound: normal echogenicity, no focal hypoechoic lesions - Imaging: evidence of bilateral obstruction with RPF mass on CT - PSA normalizes after decompression (ureteral stent or nephrostomy) and RPF treatment

True Prostate Cancer with Obstruction: - PSA often markedly elevated (>10 ng/mL; commonly >20 ng/mL) - Focal hard nodule on DRE - Transrectal ultrasound: hypoechoic lesion - Biopsy: malignant cells - No retroperitoneal mass or may have incidental mass unrelated to prostate pathology

Clinical Approach: 1. Recognize the RPF diagnosis first (imaging, biopsy) 2. Depress the ureteral obstruction (stent/nephrostomy) 3. Initiate RPF-directed treatment (steroids, immunosuppression) 4. Re-check PSA at 4-8 weeks after successful decompression 5. If PSA normalizes → obstruction-related elevation, no prostate cancer 6. If PSA remains elevated → consider prostate cancer and proceed to formal urologic evaluation

This distinction is critical to avoid unnecessary prostate biopsy and aligns with standard urology guidelines emphasizing PSA density, repeat measurements, and exclusion of benign causes before invasive testing.

Classic Presenting Symptoms

Back and Flank Pain (reported in 80-90% of patients at presentation) - Character: dull, aching, noncolicky (distinguishes from nephrolithiasis) - Location: midline or bilateral flank; may radiate to lower abdomen or groin - Quality: continuous, progressive over weeks to months - Mechanism: fibrotic mass effect on peritoneum and nerves; urinary obstruction and hydronephrosis - Pain often attributed initially to musculoskeletal disorder or misdiagnosed as renal colic

Urinary Obstruction Manifestations: - Oliguria or anuria (if acute complete obstruction) - Gradual decline in urine output - Nocturia (if partial obstruction with preserved contralateral function) - No dysuria, frequency, or urgency (important negative findings)

Constitutional Symptoms (present in 40-60%) - Malaise and fatigue - Unintentional weight loss (often 5-15 kg over months) - Low-grade fever (37.5-38.5°C; not high spiking fever typical of infection) - Night sweats (may prompt evaluation for lymphoma)

Vascular Complications from Mass Effect

Venous Obstruction (IVC or Iliac Veins): - Bilateral lower extremity edema (often asymmetric depending on degree of compression) - May be mistaken for DVT (duplex ultrasound rules out thrombosis) - Scrotal/genital edema (if internal spermatic vein involved) - Risk of thrombotic complications: true DVT can supervene on top of mechanical compression

Arterial Involvement (Aorta, Iliac Arteries): - Claudication with exertion (limb claudication if iliac involvement; aortic claudication if abdominal aorta involved) - May progress to critical limb ischemia if untreated

Renal Dysfunction Presentation

Acute Kidney Injury (most concerning presentation) - Bilateral ureteral obstruction with acute elevation in creatinine - Oliguric or non-oliguric - Hyperkalemia, metabolic acidosis, and uremia possible if severe - Requires urgent decompression (ureteral stent or percutaneous nephrostomy)

Chronic Kidney Disease (more insidious) - Gradual decline in GFR over months to years - Often diagnosed incidentally on routine laboratory screening - May progress to end-stage renal disease if obstruction not relieved

Atypical or Uncommon Presentations

• Palpable abdominal mass (rare; suggests very large inflammatory mass)
• Acute coronary syndrome-like chest pain (if mediastinal fibrosis present)
• Dysphagia (esophageal involvement)
• Fever of unknown origin (if inflammatory markers very elevated)
• Incidental finding on imaging obtained for other indications (increasingly common with widespread CT use)

Laboratory Evaluation

Markers of Inflammation: - Erythrocyte Sedimentation Rate (ESR): Elevated >80 mm/hr in ~80% of cases; nonspecific but sensitive - C-Reactive Protein (CRP): Elevated in >80%; correlates better with active inflammation than ESR in some cohorts - Both normal ESR and CRP: Do NOT exclude RPF; absence of elevation associated with more fibrotic (less inflammatory) disease phenotype

Serum IgG4: - Elevated (>135 mg/dL) in approximately 40% of idiopathic RPF cases - NOT required for diagnosis - Useful for prognostication: elevated IgG4 predicts steroid responsiveness - Repeat measurement during treatment: declining levels suggest treatment response

Renal Function and Electrolytes: - Serum creatinine: baseline assessment; elevated if bilateral obstruction present - BUN/creatinine ratio: may be elevated if obstruction superimposed on baseline CKD - Potassium: hyperkalemia if AKI from obstruction - Urine osmolality/specific gravity: may be elevated if concentrating (bladder obstruction vs. prerenal state)

Urine Analysis: - Typically normal or mildly abnormal - Absence of significant proteinuria, hematuria, or pyuria (important negative finding; presence raises concern for glomerular disease, infection, or malignancy)

Autoantibodies: - Antinuclear antibody (ANA): positive in ~60% of idiopathic RPF (nonspecific; reflects polyclonal B-cell activation) - Rheumatoid factor: may be low-positive - Anti-neutrophil cytoplasmic antibodies (ANCA): negative (if positive, consider vasculitis instead) - Anti-mitochondrial antibody: present in some IgG4-RD patients with sclerosing cholangitis

Tumor Markers (if malignancy suspected): - CEA, CA 19-9, PSA - Elevated levels increase suspicion for malignant RPF; normal levels favor idiopathic

Imaging Evaluation

Tissue Diagnosis: CT-Guided Biopsy

Indications for Biopsy: 1. Atypical imaging features (eccentric mass, invasion of adjacent structures, heterogeneous density) 2. Negative or indeterminate serum IgG4 in setting of suspected IgG4-RD 3. Concern for malignancy (lymphoma, sarcoma, metastatic disease) 4. No response to steroids after 4-8 weeks (requires exclusion of alternative diagnosis) 5. Unusual location (lateral, massive, or rapidly enlarging mass)

Biopsy Technique: - CT-guided needle biopsy: 18-gauge core needle; performed percutaneously under CT fluoroscopy - High diagnostic yield (>90% when adequate tissue obtained) - Low morbidity (<5% minor complications; <0.5% major complications like hematuria, infection) - Preferred initial approach

• Laparoscopic biopsy: If CT-guided approach technically infeasible or nondiagnostic
  • Direct visualization allows targeted sampling
  • Permits assessment of extent and involvement of surrounding structures
  • Higher morbidity than percutaneous approach
• Open surgical biopsy: Reserved for cases requiring ureterolysis (combines diagnostic and therapeutic intent)

Histopathologic Examination: - Send for: - Standard H&E histology - IgG4 immunohistochemistry (IgG4+ cell count and IgG4+/IgG ratio) - Trichrome stain (highlights collagen fibrosis) - Immunofluorescence (excludes vasculitis; if positive, suggests IgG or complement deposition)

Histology Expected in Idiopathic (IgG4-Related) RPF: - Storiform fibrosis (mandatory finding) - Dense lymphoplasmacytic infiltrate - Obliterative phlebitis - >10 IgG4+ cells/HPF; IgG4+/IgG ratio >40% - Absence of malignant cells, granulomas, or organisms

Histology Expected in Secondary RPF: - Malignancy: Cytologic atypia, increased mitotic figures, architectural disarray specific to cell type - Infection: Granulomas (TB, fungal), microorganisms on special stains - Drug-induced: Nonspecific fibrosis without IgG4 enrichment - Post-radiation: Hyalinized fibrosis, vascular hyalinization, atypical fibroblasts - Post-surgical: Foreign body reaction to graft material; nonspecific fibrosis

Acute Management: Decompression of Ureteral Obstruction

Urgent Interventions for AKI:

When RPF presents with AKI and bilateral hydronephrosis, acute decompression takes precedence over other interventions:

1. Ureteral Stent Placement (First-Line)
   • Performed via cystoscopy and retrograde passage
   • Indications: bilateral obstruction causing AKI; unilateral obstruction with solitary kidney
   • Timing: within 24-72 hours of diagnosis
   • Expected creatinine improvement: 30-50% reduction in serum creatinine within 1-2 weeks if obstruction is recent
   • Monitoring: repeat serum creatinine at 24-48 hours, then every 1-2 weeks during initial phase
   • Duration: typically 4-12 weeks during medical treatment phase
   • Complications: ureteral perforation, stent migration, infection, encrustation
   • Stent exchange every 4-6 weeks if long-term placement anticipated
2. Percutaneous Nephrostomy (Alternative or Adjunctive)
   • Indications: ureteral stent placement fails; posterior calyx anatomy unfavorable; infected obstructed system (pyonephrosis); very large stone burden
   • Performed via CT or ultrasound guidance; percutaneous puncture into renal pelvis with catheter placement
   • Allows external drainage of urine and monitoring of output
   • Useful for renal functional recovery assessment (isolated kidney function via split-function renography)
   • Advantages: bypasses ureteral stricture, allows urine culture/analysis, lower risk of infection if careful care maintained
   • Disadvantages: external catheter (patient burden, risk of dislodgement, infection)
   • May be combined with retrograde ureteral stent (antegrade-retrograde approach) in complex cases
3. Renal Function Recovery After Decompression
   • Creatinine improvement timeline: Rapid (hours to days) if acute obstruction; slower (weeks to months) if chronic obstruction with baseline CKD
   • Peak improvement at 4-12 weeks post-decompression
   • Further improvement may occur over months as inflammation resolves with medical therapy
   • Irreversible renal damage if obstruction duration >12 weeks (extensive tubular atrophy)

Medical Therapy for Idiopathic (IgG4-Related) RPF

Surgical Intervention: Ureterolysis

Indications for Ureterolysis: 1. Recurrent obstruction despite medical therapy and steroid taper completion 2. Steroid failure: No response after 8-12 weeks of adequate medical therapy 3. Steroid intolerance: Severe adverse effects precluding continued therapy 4. Fibrotic phenotype: Predominantly T2-hypointense disease on MRI (mature fibrosis less responsive to medical therapy) 5. High relapse risk: After multiple relapses despite compliance 6. Diagnostic uncertainty: Need to exclude malignancy despite imaging and biopsy

Surgical Technique: 1. Open Ureterolysis - Midline incision; careful dissection of retroperitoneal fibrosis away from ureters - Aims to free ureters from fibrotic entrapment - Assesses extent of disease; permits intraoperative biopsy if needed - Reconstruction: may require psoas hitch (mobilize bladder) or transureteroureterostomy (TUU; anastomose ureters)

2. Omental Wrap (Critical Adjunct)
   • After ureterolysis, mobilize omentum from greater curvature of stomach
   • Wrap around freed ureters to create barrier between ureters and retroperitoneal fibrosis
   • Significantly reduces re-encasement rate (from 20-30% without wrap to <5% with wrap)
   • Mechanism: vascularized tissue creates inflammatory barrier; omentum has anti-inflammatory properties
3. Laparoscopic Ureterolysis
   • Increasingly performed at specialized centers
   • Lower morbidity compared to open approach
   • Reduced postoperative pain and faster recovery
   • Technical demands higher; longer operative time
   • Omental wrap may be more challenging laparoscopically

Timing of Surgery: - After failed medical therapy (8-12 weeks adequate trial) OR early if steroid intolerance - NOT performed acutely during presentation (address acute obstruction with stent/nephrostomy first) - May be considered earlier in fibrotic-predominant disease if imaging suggests poor prognosis for medical response

Surgical Outcomes: - Success rate (durable ureteral patency): >90% with appropriate surgical technique - Re-obstruction rate: 5-15% with omental wrap; 20-30% without wrap - Complication rates: bleeding, infection, ureteral perforation, adhesions (all <5% in experienced hands)

Treatment of Secondary RPF

Malignancy-Related RPF: - Treat underlying malignancy (chemotherapy, radiation, targeted therapy, surgery as indicated) - Ureteral stent for symptom relief and renal function preservation - Surgery rarely curative as disease is systemic

Drug-Induced RPF: - Cease offending agent (if still being used; rare in modern practice given withdrawal of ergots and problematic beta-blockers) - Supportive care with ureteral stents if obstruction present - Corticosteroids: unclear evidence; may reduce inflammation but fibrosis may be irreversible - Ureterolysis: definitive treatment if severe obstruction despite medical management

Infection-Related RPF (TB, fungal): - Specific antimicrobial therapy (tuberculous: INH/RIF/PZA/EMB; fungal: amphotericin followed by azoles) - Duration: 6-24 months depending on organism - Ureterolysis: considered for recurrent obstruction after completing antimicrobial course - Steroids: generally avoided (worsens infection); exception: adjunctive low-dose steroids in severely inflammatory TB with immune reconstitution inflammatory syndrome

Radiation-Induced RPF: - No specific medical therapy - Ureteral stent for obstruction - Ureterolysis: reserved for recurrent obstruction; higher re-obstruction rate compared to idiopathic RPF

Overall Prognosis

Idiopathic (IgG4-Related) RPF: - Survival: 5-year overall survival >90% - Renal function: 50-60% maintain stable renal function with treatment; 15-20% progress to CKD stage 4-5 (often from chronic obstruction prior to diagnosis) - Treatment response: 80-90% achieve complete or partial response to medical therapy - Relapse: 20-30% relapse during or after initial treatment course

Secondary RPF: - Malignancy-related: Prognosis determined by underlying cancer; RPF indicates advanced/metastatic disease - Drug-induced: Good prognosis if drug discontinued early; may have residual fibrosis - Infection-related: Depends on organism and timing of treatment initiation - Radiation-induced: Fibrosis typically progressive; renal function slowly declines

Factors Predicting Favorable Response to Medical Therapy

1. High inflammatory phenotype (elevated ESR/CRP, elevated IgG4, T2 hyperintense on MRI)
2. Early diagnosis (shorter duration of symptoms)
3. Absence of extensive fibrosis on histology (>50% lymphocytes vs. >50% collagen fibrosis)
4. Younger age at diagnosis (<60 years)
5. Absence of other organ involvement (isolated retroperitoneal disease)

Factors Predicting Relapse Risk

1. Rapid taper of corticosteroids (>5 mg weekly reduction)
2. Multiple relapses on initial treatment course (harbinger of chronic relapsing disease)
3. Systemic IgG4-RD (multiple organ involvement)
4. Steroid-sparing agent not used during initial treatment
5. High IgG4 levels persistently elevated despite medical therapy

Long-Term Follow-Up Protocol

Duration: - Active surveillance: minimum 2 years after treatment completion; longer if high relapse risk - Indefinite surveillance if history of multiple relapses or systemic IgG4-RD

Frequency of Assessment: - Months 0-6: Every 4-8 weeks (clinical, labs, imaging) - Months 6-12: Every 8-12 weeks - Months 12-24: Every 3 months - After 24 months: Every 6-12 months (if stable on minimal/no therapy)

Components of Surveillance: 1. Clinical history: Symptom recurrence (pain, constitutional symptoms), complications 2. Physical exam: Edema, abdominal tenderness 3. Laboratory: - Serum creatinine (every 3-6 months) - ESR/CRP (every 3-6 months; more frequent if high relapse risk) - IgG4 (every 3-6 months if initially elevated) - CBC, LFTs (if on immunosuppression) 4. Imaging: - CT or MRI: at 3 and 6 months during treatment; then every 6-12 months for 2 years; then annually × 3-5 years - Alternate between CT and MRI to reduce radiation; MRI preferred for serial assessment if available - More frequent imaging (every 3 months) if relapse risk high or during treatment taper

Ureteral Stent Management During Follow-Up: - Timeline: initial stent removed at 4-8 weeks post-diagnosis after medical therapy initiated - Stent indole removal protocol: 1. Repeat imaging (CT/MRI) to confirm no residual obstruction 2. Stent removal at cystoscopy 3. Recheck serum creatinine 1-2 weeks post-stent removal (assess for any acute elevation suggesting recurrent obstruction) - Re-stenting: indicated if creatinine rises >25% post-stent removal or signs of obstruction on imaging

Screening for Other Organ Involvement in IgG4-RD

At initial diagnosis and periodically during follow-up:

1. Pancreatic imaging: Ultrasound or CT abdomen (assess for autoimmune pancreatitis)
2. Hepatic/biliary: LFTs; MRCP if elevated alkaline phosphatase or GGT (assess for sclerosing cholangitis)
3. Salivary/lacrimal: Clinical exam; imaging if symptoms (submandibular/parotid swelling, xerostomia, orbital symptoms)
4. Thyroid: TSH; thyroid ultrasound if abnormal or clinical findings
5. Aorta: Assess on CT/MRI of abdomen (look for aortic thickening/aneurysm)
6. Renal parenchyma: TIN may be present; follow proteinuria and renal function trends

Malignancy (Most Critical Exclusion)

Lymphoma: - Most common malignancy associated with retroperitoneal obstruction - Hodgkin and non-Hodgkin types both present with lymphadenopathy ± mass - Imaging: multiple nodal masses, heterogeneous enhancement, potential for lymph nodes to blend into confluent mass - Tissue diagnosis critical (malignant lymphocytes on biopsy, surface marker analysis) - Positron emission tomography (PET): high FDG avidity in active lymphoma

Sarcomas: - Liposarcoma, fibrosarcoma, leiomyosarcoma, angiosarcoma - Imaging: heterogeneous appearance, potential for invasion of adjacent structures - Often large mass at presentation - Tissue diagnosis essential (histologic malignant features)

Metastatic Carcinoma: - Breast, colon, gastric, pancreatic, prostate, lung, ovarian - Often history of known primary malignancy - Multiple nodal/soft tissue masses common - Imaging: heterogeneous, may show invasion or distant metastases

Differentiation from RPF: - Imaging features suggestive of malignancy: medial deviation of ureters, encasement without invasion favors RPF; invasion with destruction favors malignancy - Tissue diagnosis definitive - Elevated tumor markers, constitutional symptoms (though nonspecific) - History of malignancy or risk factors - Rate of progression on serial imaging (rapid growth favors malignancy)

Autoimmune/Inflammatory

Erdheim-Chester Disease: - Histiocytosis with xanthoma infiltration of retroperitoneum, mediastinum, and bone - Imaging: symmetric perivascular infiltration (characteristic “coated aorta”) - Tissue diagnosis: xanthomas with foamy macrophages - IgG4 not typical; may have elevated IgG and immune dysregulation - Often associated with CNS involvement (pituitary stalk infiltration)

Sarcoidosis: - Granulomatous inflammatory disease - Retroperitoneal involvement rare but reported - Typically mediastinal lymphadenopathy and pulmonary involvement - Biopsy: granulomas (noncaseating) - Labs: elevated ACE level, hypercalcemia, elevated IgG

Amyloidosis: - Retroperitoneal amyloid infiltration can cause obstruction - Often associated with chronic kidney disease or multiple myeloma - Tissue diagnosis: Congo red stain with apple-green birefringence; mass spectrometry - May have renal involvement (AL amyloidosis)

Infectious Causes

Tuberculosis: - Endemic regions or immunocompromised patients at higher risk - Lymphadenitis with caseating granulomas - Tissue diagnosis, culture, or nucleic acid amplification test (NAAT) - Negative TB exposure history, tuberculin skin test, or interferon-gamma release assay argues against

Actinomycosis: - Chronic suppurative infection by Actinomyces israelii (anaerobic bacterium) - Granulomatous inflammation with sinus tracts - Often history of trauma or prior procedure - Culture (anaerobic), histology showing branching filaments - Responds to prolonged antibiotics (penicillin)

Histoplasmosis: - Endemic fungi in Ohio/Mississippi river valleys - Granulomatous inflammation - Serology, antigen detection, culture - Responds to antifungal therapy

Radiation-Related

Post-Radiation Fibrosis: - History of prior abdominal/pelvic radiation (>50 Gy) - Latency 5-20 years post-radiation - Nonspecific fibrosis histologically (not IgG4-predominant) - Progressive despite treatment; prognosis determined by degree of fibrosis

Vascular Causes

Aortic Aneurysm with Inflammatory Changes: - Aortitis (bacterial, fungal, or idiopathic inflammatory aortitis including IgG4-associated aortitis) - Associated aortic thickening, periaortic halo - May cause obstruction of aorta or nearby structures - Imaging shows aortic involvement; histology if biopsied shows inflammatory infiltrate

Pearl 1: The Serum IgG4 Paradox

Clinicians often order serum IgG4 with expectation that elevation confirms IgG4-RD or RPF. This is incorrect. Approximately 60% of idiopathic RPF patients have normal serum IgG4 despite histopathologic IgG4-RD. Therefore, normal IgG4 does NOT exclude disease. Tissue diagnosis remains the gold standard.

Pearl 2: Ultrasound Misses RPF

A patient presents with bilateral hydronephrosis on renal ultrasound. Urology performs retrograde stent placement but does not obtain CT to assess retroperitoneum. The etiology (RPF, malignancy, etc.) goes undiagnosed. Always proceed to CT/MRI when obstruction of unclear etiology is identified on ultrasound. Ultrasound cannot visualize the retroperitoneal mass.

Pearl 3: PSA Is Not Prostate Cancer

Significantly elevated PSA (>20 ng/mL) in a man with bilateral obstruction and retroperitoneal mass is obstruction-related, not cancer-related. PSA normalizes after decompression and treatment of RPF. Avoid unnecessary prostate biopsy; recognize the obstruction-related PSA elevation mechanism.

Pearl 4: T2 Signal on MRI Predicts Treatability

An MRI shows an RPF mass with T2 hyperintense (bright) signal. This indicates active inflammation and excellent prognosis for steroid response. A mass with T2 hypointense (dark) signal indicates mature fibrosis; expect slower response and consider early addition of steroid-sparing agents. MRI tissue characterization guides treatment intensity.

Pearl 5: Omental Wrap Prevents Re-Obstruction

In patients proceeding to ureterolysis, omental wrap reduces re-obstruction from 20-30% (without wrap) to <5% (with wrap). This is a simple, evidence-based adjunct that markedly improves surgical outcomes.

Pearl 6: Relapse Is Common; Plan for Long-Term Follow-Up

One in four patients relapse during or shortly after initial treatment. This is not treatment failure; it is expected biology. Warn patients at treatment initiation that long-term follow-up (2+ years minimum) and possible maintenance immunosuppression are anticipated.

Pearl 7: Systemic Disease Requires Screening

IgG4-related RPF is often the presenting manifestation of systemic IgG4-RD. Screen for pancreatic (autoimmune pancreatitis), biliary (sclerosing cholangitis), thyroid, salivary, and vascular involvement at baseline and during follow-up. Systemic disease impacts prognosis and treatment decisions.

Pearl 8: Infection Must Be Excluded

RPF secondary to TB or fungal infection has different management and prognosis than idiopathic disease. Biopsy with special stains (AFB for TB, fungal stains) is indicated when: - Patient from endemic region (TB-prevalent area) - Histology shows granulomas - No response to steroids - Systemic symptoms prominent (fever, weight loss)