IgG4-related peritoneal disease is a rare extrahepatopancreatic manifestation of IgG4-related disease (IgG4-RD). IgG4-RD is a chronic fibro-inflammatory condition characterized by dense infiltration of IgG4-positive plasma cells, storiform fibrosis, obliterative phlebitis, and usually elevated serum IgG4. Peritoneal involvement may present as retroperitoneal fibrosis (most common abdominal IgG4-RD form), mesenteric panniculitis-like involvement, omental thickening, and peritoneal nodules/masses. Most patients have concurrent pancreatic (autoimmune pancreatitis), biliary (IgG4 cholangitis), or other organ involvement. Diagnosis requires elevated serum IgG4, characteristic imaging findings, and histopathological confirmation. Dramatic response to corticosteroid therapy supports the diagnosis and forms the basis of treatment.
Age Range
40-75
Peak Age
55
Gender
Male predominant
Prevalence
Rare
The pathophysiology of peritoneal involvement in IgG4-RD reflects the disease's general mechanisms in the peritoneal region. The triggering antigen has not been fully elucidated, but activation of T follicular helper (Tfh) cells and clonal expansion of IgG4-switched B cells play central roles. Activated Tfh cells secrete IL-4, IL-10, and IL-13, inducing IgG4 class switching and triggering fibroblast activation. IgG4-positive plasma cells infiltrate peritoneal tissues and storiform (basket-weave pattern) fibrosis develops — this pattern is highly characteristic of IgG4-RD. Obliterative phlebitis (occlusion of small and medium veins by fibrous infiltration) is the pathognomonic histological finding of vascular damage. In retroperitoneal fibrosis, periaortic soft tissue thickening forms from adventitial and periadventitial inflammation of the aortic wall — appearing as 'rind-like' soft tissue thickening surrounding the aorta on CT. On MRI, T2 hyperintensity (edema and cellular infiltration) and enhancement are seen in the active inflammatory phase, while T2 hypointensity (mature collagen) and decreased enhancement are seen in the fibrotic phase. This T2 signal change is an MRI indicator of disease activity and reflects treatment response.
The combination of periaortic soft tissue thickening ('rind sign') surrounding the infrarenal aorta with concurrent detection of pancreatic, biliary, or other organ involvement is a highly characteristic finding combination for IgG4-related disease and strongly supports the diagnosis. This multiorgan involvement pattern reflects the systemic fibro-inflammatory nature of IgG4-RD.
The most characteristic CT finding in IgG4-related retroperitoneal fibrosis is homogeneous soft tissue thickening surrounding the infrarenal aorta and iliac artery bifurcation ('rind sign'). Thickening is generally symmetric and well-defined, encasing the aorta anterolaterally and posterolaterally but typically sparing the posterior aortic wall ('anterior crescent sign'). The soft tissue thickening shows mild-to-moderate enhancement on contrast-enhanced CT. Ureteral obstruction (hydronephrosis) is a frequently accompanying complication.
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Homogeneous soft tissue thickening symmetrically surrounding the infrarenal aorta ('rind sign') is observed, consistent with IgG4-related retroperitoneal fibrosis; bilateral hydronephrosis is accompanying.
On T2-weighted MRI, retroperitoneal/peritoneal fibrous tissue shows variable signal depending on disease activity. In the active inflammatory phase: T2 intermediate-to-high signal (edema, cellular infiltration, loose fibrosis). In the chronic fibrotic phase: T2 low signal (mature collagen, dense fibrosis). This T2 signal variation is an MRI indicator of disease activity and critical for treatment response evaluation. Prominent gadolinium enhancement accompanies the active phase, decreased enhancement the fibrotic phase.
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Retroperitoneal/peritoneal fibrous tissue demonstrates intermediate-to-high signal on T2-weighted MRI, consistent with active inflammatory phase; response to corticosteroid therapy is expected.
In the IgG4-related mesenteric panniculitis pattern, increased density (hazy mesenteric fat — 'misty mesentery') and soft tissue infiltration around the mesenteric root and branches is observed. 'Fat ring sign' — preserved fat halo around mesenteric vessels — may be seen and is typical for mesenteric panniculitis. Peritoneal thickening and omental nodular infiltration may accompany. Multiple organ involvement (pancreas, biliary, lacrimal, salivary glands) supports IgG4-RD diagnosis.
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Increased density ('misty mesentery') and soft tissue infiltration around the mesenteric root with accompanying fat ring sign; IgG4-related mesenteric panniculitis should be considered.
On contrast-enhanced MRI, IgG4-related peritoneal/retroperitoneal lesions show variable enhancement depending on disease activity. In the active phase, prominent homogeneous gadolinium enhancement is observed — reflecting increased vascularity and capillary permeability of inflammatory tissue. In the fibrotic phase, enhancement decreases or remains minimal. Delayed phase persistent enhancement reflects contrast retention by fibrous tissue. This enhancement pattern guides treatment decisions and monitors disease activity during follow-up.
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Retroperitoneal/peritoneal lesions demonstrate prominent gadolinium enhancement on contrast-enhanced MRI, consistent with active inflammatory phase.
Multiorgan involvement of IgG4-RD provides important diagnostic clues. Most commonly accompanying findings: (1) Pancreas: diffuse enlargement ('sausage pancreas'), peripheral hypointense halo ('halo sign'), peripancreatic soft tissue thickening; (2) Biliary tract: diffuse or focal wall thickening, biliary strictures; (3) Salivary/lacrimal glands: bilateral symmetric enlargement; (4) Periaortic lymphadenopathy; (5) Kidneys: bilateral cortical nodules or diffuse parenchymal infiltration. Concurrent detection of multiple organ involvement on CT/MRI strongly supports IgG4-RD diagnosis.
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In addition to peritoneal/retroperitoneal involvement, diffuse pancreatic enlargement and biliary wall thickening are observed, consistent with multiorgan IgG4-related disease.
Active IgG4-RD lesions show mild-to-moderate FDG uptake on FDG PET-CT (SUVmax typically 3-8). PET-CT is particularly valuable in assessing disease extent (detecting unexpected organ involvement), guiding biopsy site, and monitoring treatment response. Decreased FDG uptake after corticosteroid therapy confirms treatment response. Intense FDG uptake (SUVmax >10) should raise concern for malignancy.
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Mild-to-moderate FDG uptake in retroperitoneal/peritoneal lesions and accompanying organ involvement on PET-CT, consistent with active IgG4-related disease.
Criteria
Periaortic soft tissue thickening around infrarenal aorta and iliac arteries, ureteral obstruction, elevated serum IgG4, storiform fibrosis and IgG4+ plasma cells on histology.
Distinct Features
30-60% of retroperitoneal fibrosis is IgG4-related. Dramatic response to corticosteroids. Bilateral hydronephrosis common complication. May be associated with aortic aneurysm/dissection. Relapse 20-40% upon steroid cessation.
Criteria
Mesenteric fat infiltration, 'misty mesentery' and fat ring sign findings, elevated serum IgG4, IgG4+ plasma cell infiltration with storiform fibrosis on biopsy.
Distinct Features
Distinction from idiopathic mesenteric panniculitis is important — treatment differs. IgG4-related form has good steroid response. May overlap with sclerosing mesenteritis. Multiorgan screening mandatory.
Criteria
Nodular thickening and masses on peritoneal surfaces, omental thickening, ascites may accompany. May mimic peritoneal carcinomatosis or tuberculous peritonitis. IgG4-RD histological criteria on biopsy.
Distinct Features
Rarest peritoneal IgG4-RD form. Differential diagnosis with peritoneal carcinomatosis is critical — avoid misdiagnosis of malignancy and unnecessary treatment. Laparoscopic biopsy usually needed. Good steroid response.
Distinguishing Feature
Peritoneal carcinomatosis usually presents with primary malignancy history, peritoneal nodules are more prominent and asymmetric with diffuse omental cake. IgG4-RD peritoneal involvement is more diffuse and symmetric, serum IgG4 elevated, multiorgan involvement accompanies, and dramatic steroid response.
Distinguishing Feature
Tuberculous peritonitis shows rim-enhancing lymph nodes, high-density ascites, and peritoneal thickening. IgG4-RD does not show rim-enhancing nodes or high-density ascites; multiorgan involvement pattern and elevated serum IgG4 are distinguishing.
Distinguishing Feature
Idiopathic sclerosing mesenteritis may overlap with IgG4-RD; distinction relies on serum IgG4, IgG4+ plasma cell count and IgG4/IgG ratio on biopsy. Idiopathic form has less prominent steroid response and multiorgan involvement is not expected.
Distinguishing Feature
Peritoneal lymphomatosis typically shows more intense FDG uptake (SUVmax >10), peripheral blood count anomalies, and bone marrow involvement. IgG4-RD shows lower FDG uptake (SUVmax <8), elevated serum IgG4, and responds to steroids.
Urgency
routineManagement
medicalBiopsy
NeededFollow-up
specialist-referralTreatment of IgG4-related peritoneal disease starts with corticosteroids with generally dramatic response. Prednisolone 0.6 mg/kg/day initial dose for 2-4 weeks followed by gradual taper (over 3-6 months). Rituximab (anti-CD20) is effective as second-line therapy in steroid-refractory or relapsing cases. Ureteral obstruction may require urgent stenting. Treatment response is monitored by serum IgG4 levels, imaging (lesion regression on CT/MRI), and clinical symptoms. Relapse occurs in 20-40% and may require long-term low-dose maintenance therapy. In peritoneal forms mimicking malignancy, histopathological confirmation is mandatory — avoid misdiagnosis of malignancy and unnecessary cytoreductive surgery.
IgG4-related mesenteritis shows dramatic response to steroid therapy. Multi-organ involvement should be screened (pancreas, biliary tract, salivary glands, kidney). Serum IgG4 level is used for diagnosis and follow-up. Histologic confirmation with biopsy is recommended.