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Hepatobiliary and Pancreatic: Pancreatic cancer with elevated serum IgG4 level due to multiple myeloma mimicking localized autoimmune pancreatitis

An 80-year-old Japanese woman was referred for examination of a pancreas head mass lesion. She was diagnosed with multiple myeloma (MM) at another hospital 2 years before referral. Laboratory tests revealed abnormal liver function test and elevated serum IgG, IgG4, (1560 mg/dL) and CA19-9 levels (154.2 U/mL). Initial computed tomography (CT) (Fig. 1a, arrow) and endoscopic ultrasonography (EUS) (Fig. 1b, arrowheads) showed a mass in the pancreas head that involved the distal bile duct (BD). EUS guided fine-needle aspiration of the mass found no evidence of malignancy or autoimmune pancreatitis (AIP). The biopsy specimen from the distal BD showed infiltration of few plasma cells without IgG4 staining. Based on the diagnosis of localized type 1 AIP by international consensus diagnostic criteria (indeterminate imaging, level 1S and level OOI), we started to administrate prednisolone at a dose of 30 mg/day. Eight weeks after prednisolone start, the serum CA19-9 level was sharply elevated (465 U/mL), whereas serum IgG and IgG4 levels had not decreased. Follow-up CT showed expansion of the pancreas head mass. Repeated biopsy of the distal BD stricture was performed and revealed adenocarcinoma (Fig. 2a, hematoxiline and eosin stain). According to the report from the previous hospital, this patient was diagnosed with IgG-type MM (Durie-salmon staging system; clinical stage II) from laboratory test results (IgG, 2752 mg/dL; IgA, 46 mg/dL; IgM, 30 mg/dL; κ/λ ratio, 8.28; Hb, 9.0 g/dL), bone marrow findings (Fig. 2b, hematoxiline and eosin stain; Fig. 2c,d, CD138), and serum immunoelectrophoresis, which showed a clear M peak on IgG. The previous and current serum laboratory findings (IgG, 2192 mg/dL; IgG4, 1560 mg/dL; κ/λ ratio, 8.97) indicated an increase in serum monoclonal IgG4. The final diagnosis was pancreatic ductal adenocarcinoma (PDAC) complicated by MM with increased monoclonal IgG4. The patient underwent best supportive care because of her poor performance status. In the two previous studies analyzing IgG subclass distribution in patients with MM, the proportion of patients with IgG4 M-protein was extremely low (6.5% and 8%, respectively). There has been no previous report of a case of PDAC complicated by MM with IgG4 M-protein. We described an extraordinary case of PDAC that was initially misdiagnosed as localized AIP because of a markedly elevated serum IgG4 level affected by MM. Physicians should consider that the presence of MM with IgG4 M-protein could hinder a correct diagnosis of a pancreatic mass lesion.