Autoimmune pancreatitis
Overview
- Chronic fibroinflammatory pancreatitis with characteristic imaging and histology
- Two subtypes:
- Type 1: part of systemic IgG4-related disease (older males, multiorgan)
- Type 2: idiopathic duct-centric (younger, often isolated, associated with inflammatory bowel disease)
- Excellent response to steroids (diagnostic and therapeutic)
- Key challenge: distinguish from pancreatic ductal adenocarcinoma (esp. in focal type)
Epidemiology
- Type 1: middle-aged to elderly males predominant
- Type 2: younger patients, equal sex distribution
- Increasing recognition worldwide, especially in Asia and Western countries
- Extrapancreatic manifestations common in type 1 (biliary, salivary, renal, retroperitoneal)
Key radiological manifestations
CT (modality of choice)
- Diffuse pancreatic enlargement ("sausage-like") with loss of lobular contour
- Featureless appearance
- Delayed homogeneous enhancement
- Characteristic low-attenuation capsule-like rim (hypodense halo)
- Focal form: mass-like enlargement (head most common) mimicking carcinoma
- Narrowing or stricture of intrapancreatic common bile duct (smooth tapering)
- Minimal upstream pancreatic duct dilatation despite mass effect
MRI
- T1WI: hypointense pancreas
- T2WI: mildly hypointense with capsule-like rim (low signal)
- DWI: restricted diffusion (similar to carcinoma)
- Post-contrast: delayed enhancement pattern
- Capsule-like rim enhances late
MRCP / ERCP
- Irregular narrowing of main pancreatic duct (multiple strictures without upstream dilatation)
- Side branch involvement less prominent than in ordinary chronic pancreatitis
- Type 2 may show more duct-centric changes
PET/CT
- Intense FDG uptake in pancreas ± extrapancreatic sites (salivary glands, biliary tree, retroperitoneum)
- Useful for detecting multifocal IgG4 disease and assessing steroid response
Classic triad (type 1)
Sausage-shaped pancreas + capsule-like rim + delayed enhancement → highly suggestive of AIP in correct clinical context.
Differential diagnosis
| Diagnosis | Key differentiators |
|---|---|
| Pancreatic ductal adenocarcinoma | Irregular mass, upstream duct dilatation >5 mm, vascular encasement, metastases |
| Acute pancreatitis | Peripancreatic fluid, necrosis, clinical acute presentation |
| Chronic pancreatitis (alcohol) | Parenchymal calcification, duct stones, marked side branch ectasia |
| Groove pancreatitis | Sheet-like mass in pancreaticoduodenal groove, duodenal stenosis, cysts |
| Lymphoma | Bulky homogeneous mass, lymphadenopathy below renal hilum |
Common pitfall
Focal AIP in pancreatic head frequently mimics carcinoma → leads to unnecessary resection. Look for capsule-like rim, absent upstream duct dilatation, and multifocal IgG4 involvement.
Imaging approach
- Initial: contrast-enhanced multidetector CT (pancreatic + portal venous phases)
- Suspicious mass with atypical features → MRI + MRCP
- Systemic disease suspected → FDG-PET/CT
- Follow-up after steroids: rapid resolution of enlargement and enhancement (often within weeks)
- Biopsy if imaging equivocal (IgG4 staining essential)
Pearls
- Absence of significant upstream pancreatic duct dilatation despite mass effect → strong clue against carcinoma
- Extrapancreatic findings (bilateral salivary enlargement, renal lesions, retroperitoneal fibrosis) virtually diagnostic of type 1
- Steroid trial response (clinical + imaging) often confirmatory
- Type 2 less likely to have capsule rim or systemic features
- Report should suggest AIP when imaging features present and recommend serum IgG4 + steroid trial if appropriate
Reporting tip
Phrase as "features suggestive of autoimmune pancreatitis (capsule-like rim, delayed enhancement, no significant duct dilatation); recommend correlation with serum IgG4 and consider steroid trial to avoid unnecessary surgery."
