OBJECTIVETo investigate clinicopathological features of fibrous mass-forming chronic pancreatitis (FMCP), to compare clinicopathological and immunohistochemical characteristics between autoimmune pancreatitis (AIP) and fibrous mass-forming non-autoimmune pancreatitis (nAIP) and to provide evidence for pathological diagnosis, differential diagnosis and clinical treatment strategy.

METHODSClinicopathological features were analyzed in 81 cases of FMCP. Infiltrating IgG4(+) plasmacytes were counted by immunohistochemical staining.

RESULTSAmong 81 cases of FMCP, 20 cases were diagnosed as AIP and 61 cases were interpreted as nAIP. AIP was more common in males over 50 years, whereas nAIP was seen in much younger patients (P = 0.001). The amount of inflammatory cells in the stroma of AIPs was remarkable higher than that in nAIPs (P = 0.002). The incidence of neuritis in AIPs (100%, 20/20) was also higher compared with that of nAIPs (75.4%, 46/61; P = 0.017). Storiformed-fibrosis was more common in AIPs (95.0%, 19/20) than in nAIPs (1.6%, 1/61;P = 0.000). Pancreatic intraepithelial neoplasia (PanIN) was observed in 50.0%(10/20) of AIPs and 32.8%(20/61) of nAIPs, with a greater severity observed in AIPs (P = 0.031). Tubular complex (TC) was more commonly observed in AIPs (65.0%, 13/20) than nAIPs (26.2%, 16/61;P = 0.002). Among 81 cases of FMCP, 61 cases had less than 11 IgG4(+) plasmacytes /HPF, 7 cases had 10-30/HPF and 13 cases had over 30/HPF.

CONCLUSIONSFMCPs include both AIP and nAIP. AIP has distinct pathological features and the presence of IgG4(+) plasmacyte is an important diagnostic parameter. FMCP appears to be an important precancerous lesion of pancreatic ductal adenocarcinoma. Surgery may be considered for patients with FMCP due to its mass-forming nature. In contrast, patients with AIP are treated medically due to its steroid-responsiveness. Therefore, accurate and timely diagnosis of AIP is of clinical relevance to avoid unnecessary surgical complications and to prevent progression of the disease.