Autoimmune pancreatitis: Multimodality non-invasive imaging diagnosis

Abstract

Autoimmune pancreatitis (AIP) is characterized by obstructive jaundice, a dramatic clinical response to steroids and pathologically by a lymphoplasmacytic infiltrate, with or without a pancreatic mass. Type 1 AIP is the pancreatic manifestation of an IgG4-related systemic disease and is characterized by elevated IgG4 serum levels, infiltration of IgG4-positive plasma cells and extrapancreatic lesions. Type 2 AIP usually has none or very few IgG4-positive plasma cells, no serum IgG4 elevation and appears to be a pancreas-specific disorder without extrapancreatic involvement. AIP is diagnosed in approximately 2%-6% of patients that undergo pancreatic resection for suspected pancreatic cancer. There are three patterns of autoimmune pancreatitis: diffuse disease is the most common type, with a diffuse, "sausage-like" pancreatic enlargement with sharp margins and loss of the lobular contours; focal disease is less common and manifests as a focal mass, often within the pancreatic head, mimicking a pancreatic malignancy. Multifocal involvement can also occur. In this paper we describe the features of AIP at ultrasonography, computed tomography, magnetic resonance and positron emission tomography/computed tomography imaging, focusing on diagnosis and differential diagnosis with pancreatic ductal adenocarcinoma. It is of utmost importance to make an early correct differential diagnosis between these two diseases in order to identify the optimal therapeutic strategy and to avoid unnecessary laparotomy or pancreatic resection in AIP patients. Non-invasive imaging plays also an important role in therapy monitoring, in follow-up and in early identification of disease recurrence.

Keywords: Autoimmune pancreatitis; Computed tomography; Magnetic resonance; Pancreatic imaging; Ultrasonography.

Figure 1

Diffuse-type autoimmune pancreatitis. A-H: Computed tomography: the pancreas appears diffusely enlarged (arrows in A-D) with a hypodense peripancreatic rim, better visible in the venous phase (arrow in E). The lesion shows fair enhancement resulting almost isodense in the delayed phase (G-H). A plastic biliary endoprothesis is visible in the common bile duct (arrow in H); I-O: Magnetic resonance: the entire organ is slightly hypointense on T1-weighted images (arrow in I) and slightly hyperintense on T2-weighted images (arrow in J), with diffusion coefficient restriction (arrows in K and L) with intermediate-high b values. At dynamic examination the pancreatic lesion presents fair enhancement resulting almost isodense in the delayed phase (arrow in O).

Figure 2

Focal-type autoimmune pancreatitis. A-C: Computed tomography: the body of the pancreas appears focally enlarged (arrow in A) with a hypodense peripancreatic rim, better visible in the venous phase (arrow in B). The lesion shows fair enhancement resulting almost isodense in the delayed phase (arrow in C); D-K: Magnetic resonance: the affected portion of the pancreas is slightly hypointense on T1-weighted fat-saturated (arrow in D) images and slightly hyperintense on T2-weighted fat-saturated images (E), with diffusion coefficient restriction (arrows in F-G) with intermediate-high b values. At dynamic examination the pancreatic lesion shows fair enhancement resulting almost isodense in the delayed phase (arrow in J). At magnetic resonance cholangiopancr-eatography the main pancreatic duct shows a focal stenosis (long arrow in K) without upstream dilation. The intrahepatic bile ducts present irregular slightly stenotic portions (short arrows in K), due to involvement in the autoimmune process.