2020 Volume 35 Issue 4 Pages 272-279
For the diagnosis of type 1 autoimmune pancreatitis (AIP) by biopsy, it is important to understand the histological features of capsule-like lesions in the peripheral pancreas and lobular lesions. A capsule-like lesion in the peripheral pancreas is likely to be inflamed adipose tissue. Storiform fibrosis is observed frequently, but not always. These lesions may also consist of simple lymphoplasmacytic infiltration or fibrosis. The lobular contour is preserved in AIP, but acinar cells in the lobules are markedly decreased in number and are replaced by inflammatory cells and acinar-ductal metaplasia (ADM). Storiform fibrosis may be present, and the lobules tend to be enlarged. Atrophic and fibrotic lobules are also focally admixed. It is important to distinguish ADM from pancreatic cancers, and the concept of ADM should be acknowledged. Obliterative phlebitis, perineural inflammation, and periarteritis are also characteristic features of AIP. Numerous IgG4-positive plasma cells are seen (>10/high-power field), and a diffuse distribution of IgG4-positive cells and a high IgG4/IgG-positive cell ratio (>40%) are also helpful in establishing the diagnosis.
