Pancreatic ductal adenocarcinoma with autoimmune pancreatitis-like histologic and immunohistochemical features.

Autoimmune pancreatitis (AIP) often manifests as a mass lesion causing obstructive jaundice, clinically mimicking pancreatic carcinoma. A diagnosis of AIP may obviate the need for surgical resection, as most patients respond to steroid treatment. However, it is not clear whether these 2 conditions can coexist. In this study, 105 specimens resected for pancreatic ductal adenocarcinoma (PDAC) that also have changes of chronic pancreatitis were examined for features considered to be characteristic of AIP. Of 105 cases of PDAC with changes of chronic pancreatitis, 10 (9.5%) exhibited histologic features of AIP, including exuberant fibrosis, lymphoplasmacytic infiltration, obliterative phlebitis, or granulocytic epithelial lesions. Of these 10 cases, 7 had more than 20 immunoglobulin G4+ plasma cells per high-power field. Of these 7 cases, 5 were analyzed for Kirsten rat sarcoma viral oncogene mutation and SMAD4 expression. Three cases showed K-ras mutation and/or loss of SMAD4 expression in benign AIP-like areas. These findings suggest 2 possibilities: first, AIP-like lesions may occur in a small but significant portion of PDAC cases; second, some PDACs may arise in a background of AIP. Therefore, caution is necessary when making a diagnosis of AIP by needle biopsy of a mass lesion, and patients with a tentative AIP diagnosis should be closely followed up clinically.