Utility of diffusion-weighted MRI for assessment of response to therapy in a case of autoimmune pancreatitis

Abstract

A male in his 40's was admitted to our hospital due to upper abdominal discomfort for 1 week. Blood chemistry suggested inflammation and slightly elevated levels of pancreatic enzymes. CT revealed a focal tumor in the pancreatic tail and a slight contrast enhancement in the late phase. EUS showed a low echoic tumor 43mm×27mm in size. MRI revealed the tumor to have low intensity in T1WI, high intensity in T2WI, and higher imaging in DWI. ERCP showed main pancreatic ductal narrowing over 20mm in the pancreatic tail. EUS-FNA revealed no malignancy or chronic inflammation. Serum IgG4 was elevated (432mg/dl). The final diagnosis was autoimmune pancreatitis with focal enlargement of the pancreatic tail, and steroid therapy was initiated. After one week, diffusion-weighted MRI revealed that the high-density area in the pancreatic tail was smaller and lower in intensity. The apparent diffusion coefficiency (ADC) was reduced from 1.25×10^-3mm²/s to 1.05×10^-3mm²/s. After 2 weeks, pancreatic tail swelling and main pancreatic duct narrowing were improved.