Answer: Autoimmune pancreatitis

Histology: The pancreatic parenchyma is obscured by an intense inflammatory infiltrate. While this infiltrate is worrisome for lymphoma at low-power, it is clear that the infiltrate is mixed at high magnification. The infiltrate is fairly diffuse, but there is some duct-centricity to it. In addition, a dramatic venulitis can be appreciated in the elastin stain. An immunostain for IgG4 labels numerous cells.

Discussion: Autoimmune pancreatitis can clinically mimic pancreatic cancer. Both lesions can produce painless jaundice and a mass lesion. Autoimmune pancreatitis is characterized by a duct centric mixed inflammatory cell infiltrate. Typically there is also a venulitis. The best place to look for the venulitis is at the leading edge of the inflammation. As in this case, the venulitis can be highlighted with an elastin stain. An immunostain for IgG4 can be used to confirm the diagnosis, as most cases of autoimmune pancreatitis have >20 IgG positive cells per high power field.