Autoimmune pancreatitis (AIP) is a chronic inflammatory disease of the pancreas and the most common organ manifestation of IgG4-related disease spectrum. It is characterized by diffuse or focal pancreatic enlargement. Type 1 (IgG4-related) is part of a systemic disease predominantly affecting older males; Type 2 is limited to the pancreas and associated with inflammatory bowel disease. The characteristic 'sausage-shaped pancreas' appearance, capsule-like rim, and steroid responsiveness strongly support the diagnosis. Differentiation from ductal adenocarcinoma is of critical importance.
Age Range
50-75
Peak Age
60
Gender
Male predominant
Prevalence
Rare
In Type 1 AIP, IgG4-positive plasma cells and T lymphocytes infiltrate the pancreatic parenchyma, causing dense lymphoplasmacytic inflammation and storiform (whorling) fibrosis. This infiltration leads to diffuse swelling and loss of lobulation of the pancreas, creating the characteristic 'sausage-shaped' appearance on imaging. The periparenchymal fibrous capsule-like rim demonstrates delayed contrast enhancement on CT and MRI — this delayed enhancement reflects the slow contrast entry and late washout of fibrous tissue. The pancreatic duct shows irregular narrowing without significant upstream dilatation (unlike adenocarcinoma). In Type 2, granulocyte epithelial lesions (GEL) are predominant and IgG4 elevation is typically absent. Steroid therapy dramatically reverses the inflammatory infiltration, and this therapeutic response has both diagnostic and prognostic significance.
Diffuse enlargement of the pancreas with loss of normal lobular contour resulting in a smooth, featureless 'sausage-shaped' appearance. The loss of lobulation results from lymphoplasmacytic infiltration completely filling the interlobular fat. This finding, combined with a capsule-like periparenchymal rim, is considered pathognomonic for AIP.
The pancreas is diffusely enlarged with loss of normal lobular contour. The pancreas demonstrates a smooth, featureless 'sausage-shaped' appearance. Mild hypoattenuation of the parenchyma may be detected.
Report Sentence
The pancreas is diffusely enlarged with loss of normal lobular contour demonstrating a smooth 'sausage-shaped' appearance, consistent with autoimmune pancreatitis.
In the arterial phase, the pancreatic parenchyma appears markedly hypoattenuating compared to normal parenchyma. In diffuse involvement, the entire organ enhances homogeneously but poorly; in the focal form, a mass-like hypoattenuating area is identified.
Report Sentence
In the arterial phase, the pancreatic parenchyma appears diffusely hypoattenuating with decreased enhancement.
On delayed phase, a thin, smoothly enhancing capsule-like rim (periparenchymal halo) is seen surrounding the pancreas. This rim may encircle the entire pancreas or a portion of it. Parenchymal enhancement also progressively increases on delayed phase.
Report Sentence
A thin capsule-like enhancing rim (periparenchymal halo) is seen surrounding the pancreas on delayed phase, which is pathognomonic for autoimmune pancreatitis.
On T2-weighted sequences, the pancreatic parenchyma appears diffusely mildly hypointense. While normal pancreatic T2 signal is similar to the spleen, signal is notably decreased in AIP. The capsule-like rim may be visible as a hypointense band on T2.
Report Sentence
The pancreatic parenchyma appears diffusely hypointense on T2-weighted sequences, consistent with a fibrotic inflammatory process.
On DWI, the pancreatic parenchyma shows diffusely high signal with low ADC values on ADC mapping. Diffusion restriction indicates active inflammatory infiltration and is useful in assessing treatment response.
Report Sentence
The pancreatic parenchyma demonstrates diffuse diffusion restriction on DWI, consistent with an active inflammatory process.
On T1-weighted sequences, the pancreatic parenchyma appears diffusely hypointense compared to the normally high signal of the pancreatic parenchyma. The normal high T1 signal of the pancreas arises from protein-rich acinar cells, which are replaced by inflammatory tissue in AIP.
Report Sentence
The pancreatic parenchyma appears diffusely hypointense on T1-weighted sequences, suggesting loss of normal acinar structure.
On delayed phase (3-5 minutes), the pancreatic parenchyma shows progressively increasing enhancement. The parenchyma that was hypointense on early phase demonstrates significant enhancement on delayed phase. The capsule-like rim is also most conspicuous on delayed phase.
Report Sentence
The pancreatic parenchyma demonstrates progressive enhancement on delayed phase, consistent with a fibrotic inflammatory process.
On B-mode ultrasonography, the pancreas is diffusely hypoechoic and enlarged. The normal lobular echogenicity pattern is lost. Edema and infiltration may be seen in peripancreatic fat tissue.
Report Sentence
The pancreas appears diffusely hypoechoic and enlarged on ultrasonography, consistent with an inflammatory process.
Criteria
Elevated serum IgG4 (>135 mg/dL), systemic organ involvement (bile ducts, salivary glands, retroperitoneum, kidneys), older male predominance (>60 years), lymphoplasmacytic sclerosing pancreatitis (LPSP) pattern on histology
Distinct Features
Diffuse form is more common (60-70%). Accompanied by systemic IgG4-related findings: sclerosing cholangitis (biliary stricture), retroperitoneal fibrosis, interstitial nephritis, dacryoadenitis/sialadenitis (Mikulicz disease). CT may show bilateral renal hypoattenuating lesions and retroperitoneal fibrosis.
Criteria
Normal serum IgG4, no systemic organ involvement, affects younger patients (30-40 years), strong association with inflammatory bowel disease (IBD) (20-30%), granulocyte epithelial lesion (GEL) on histology
Distinct Features
Focal form is more common and head involvement may mimic adenocarcinoma. IgG4 elevation is absent; diagnosis is established histologically. Steroid response is as good as Type 1. Bowel symptoms may accompany due to IBD association. Relapse rate is lower than Type 1.
Criteria
Focal mass form involving a segment of the pancreas, usually head or tail involvement, constitutes 30-40% of all AIP cases
Distinct Features
The most difficult form to distinguish from ductal adenocarcinoma. May show focal hypoattenuating mass, mild duct narrowing, and minimal upstream atrophy. Distinguishing clues: absence of vascular invasion, presence of capsule-like rim, marked diffusion restriction on DWI, homogeneous delayed enhancement, and IgG4 elevation. If diagnostic uncertainty persists, steroid trial or biopsy is required.
Criteria
Diffuse involvement of the entire pancreas, 'sausage-shaped pancreas' appearance, constitutes 60-70% of all AIP cases
Distinct Features
Diagnosis is usually easier with the classic sausage-shaped appearance. The duct is diffusely irregularly narrowed. The capsule-like rim surrounds the entire pancreas. Systemic IgG4 findings frequently accompany. Likelihood of confusion with ductal adenocarcinoma is lower compared to focal form.
Distinguishing Feature
Adenocarcinoma shows significant upstream pancreatic duct dilatation and parenchymal atrophy; in AIP, despite duct narrowing, significant upstream dilatation is absent. Adenocarcinoma demonstrates vascular invasion (SMA/SMV/celiac/portal vein) while AIP lacks vascular invasion. Capsule-like rim is not seen in adenocarcinoma.
Distinguishing Feature
Lymphoma typically presents as a large, homogeneous hypoattenuating mass that encases vessels without invading them. In AIP, diffuse enlargement, capsule-like rim, and delayed enhancement are prominent. IgG4 elevation is not expected in lymphoma and steroid response is temporary.
Distinguishing Feature
Focal pancreatitis has a history of acute pancreatitis with prominent peripancreatic inflammatory changes (fat stranding, fluid collections). In AIP, peripancreatic inflammation is minimal and capsule-like rim is characteristic. Diffusion restriction on DWI is less pronounced in focal pancreatitis.
Distinguishing Feature
Groove pancreatitis is localized to the groove region between the pancreatic head and duodenum with duodenal wall cystic changes. In AIP, involvement is diffuse or focal without expected duodenal cystic changes. Alcohol history is common in groove pancreatitis.
Urgency
urgentManagement
medicalBiopsy
Not NeededFollow-up
3-monthAIP diagnosis is established by HISORt criteria (histology, imaging, serology, other organ involvement, steroid response). In Type 1, serum IgG4 >135 mg/dL supports the diagnosis. First-line treatment is prednisolone (0.6-1 mg/kg/day, 2-4 weeks, then gradual taper) with >95% response rate. Steroid response is both diagnostic and therapeutic. Relapse occurs in 30-50% and may require immunomodulators (azathioprine) or rituximab. In focal form, if adenocarcinoma cannot be excluded, EUS-FNA or surgical exploration is required. Treatment response is assessed with CT/MRI at 2-4 weeks.
AIP is a benign condition that responds dramatically to steroid therapy. Differentiation from PDAC is critically important because the treatment approach is completely different (steroid vs surgery). Elevated serum IgG4 supports the diagnosis. It has two subtypes: Type 1 (IgG4-related) and Type 2 (GEL-associated). Other organ involvement (biliary, retroperitoneal fibrosis) may accompany.