It is an interesting matter how many patients with advanced-stage AIP who demonstrated definite chronic pancreatitis symptoms were included with patients having ordinary chronic pancreatitis before the proposal of AIP. We previously reported that more than 60 % of AIP patients maintain high serum IgG4 concentrations even after the clinical symptoms have resolved [21]. Accordingly, if those with advanced-stage AIP are present among cases of confirmed ordinary chronic pancreatitis, these AIP patients might maintain serum IgG4 elevation even at chronic or advanced stages. Thus, we measured serum values of IgG4 in 175 patients with confirmed chronic pancreatitis who had been diagnosed before 1995, the year when the concept of AIP was first proposed. We found high serum IgG4 concentrations in 13 patients with confirmed chronic pancreatitis (7.4 %) (12 men and 1 woman; diagnosis of 9 alcoholic and 4 idiopathic). Among the 13 patients, three had been diagnosed with pancreatic cancer at first, and one had been recently diagnosed with AIP after long-term clinical follow-up. The remaining nine patients showed typical image findings consistent with ordinary chronic pancreatitis, such as pancreatic stones and irregular dilatation of the MPD [7]. This result suggested that an advanced stage of AIP might have been previously included among cases of ordinary chronic pancreatitis and might be regarded as another condition, such as chronic alcoholic pancreatitis.

Our observation that ordinary chronic pancreatitis cases might include a few advanced-stage AIP cases and that some patients with AIP show severe pancreatic calcifications over a long-term course suggested that AIP could progress to chronic pancreatitis that meets the definitive diagnostic criteria for ordinary chronic pancreatitis. We examined whether AIP could transform into chronic pancreatitis that met the revised Japanese clinical diagnostic criteria [22] and aimed to clarify the susceptibility factors and underlying mechanisms for AIP progressing to confirmed chronic pancreatitis [23]. We enrolled 73 patients with AIP who had been followed for at least 3 years (median follow-up period, 88 months; range, 36–230 months) and found that 16 patients (22 %) were confirmed to have chronic pancreatitis in terms of the revised Japanese clinical diagnostic criteria [22], including 15 patients with definite chronic pancreatitis and one patient with probable chronic pancreatitis during this study period (Table 25.3). Among the 15 definite cases, major imaging findings were stones in pancreatic ducts in nine (Fig. 25.2a) and multiple or numerous calcifications distributed throughout the entire pancreas in 13 patients (Fig. 25.2b) [23].