Autoimmune pancreatitis (AIP) resembles pancreatic carcinoma clinically and radiographically,and as such the diagnosis of autoimmune pancreatitis is challenging to make.
This particular form of pancreatitis that was first described in 1995 and has more recently been recognized as a manifestation of IgG4-RSD. It most commonly occurs in older men. As it is commonly associated with a biliary stricture, AIP often mimics pancreatic cancer presenting with painless jaundice, weight loss, diabetes mellitus. Patients with AIP may have a focal mass or diffuse pancreatic abnormality. In the USA, AIP has been reported to account for 2.5% of all cases undergoing Whipple’s resection, and for up to 20-23% of those with benign conditions¹. AIP is commonly associated with sclerosing lesions in many other organs. Only 44% of cases of AIP have elevated IgG4-levels².

The most important differentiation from AIP is pancreatic carcinoma. Features that are highly suggestive of AIP over carcinoma are¹: diffuse enlargement of the pancreas with a capsule-like rim, and diffuse narrowing of the pancreatic duct. In AIP, areas of restricted diffusion on MRI scan can be diffuse, solitary or multiple, whereas pancreatic cancer typically shows only solitary abnormality. A serum IgG4 level of more than 2.8 g/l is highly suggestive of AIP³. 

PEt-CT has been suggested as an aid in differentiating AIP from pancreatic carcinoma^{4, 5} and in this case it was negative, i.e. did not show any significant metabolic activity in the pancreatic mass.