Our ability to recognize AIP and differentiate it from pancreatic adenocarcinoma is aided by the use of formal Mayo Clinic, Japanese and Korean diagnostic criteria [8–10]. The Mayo Clinic HISORt criteria comprise five cardinal features involving findings regarding histology, imaging, serology, other organ involvement and response to corticosteroid therapy. In AIP, the pancreas is diffusely enlarged. The predominant histologic feature is infiltration of the peri-ductal space with plasma cells and T lymphocytes. In addition to this infiltrate, acinar destruction, obliterative fibrosis involving the major and minor veins and storiform fibrosis of the pancreatic parenchyma are present. The fibrosis may extend from the pancreas to contiguous peri-pancreatic soft tissue.
Various imaging modalities, including abdominal ultrasonography, CT, ERCP, EUS, magnetic resonance imaging (MRI) and magnetic resonance cholangiopancreatography may be used in the diagnosis of AIP. On CT scans, a diffusely enlarged pancreas with the appearance of a sausage-shaped organ is a classic marker for AIP. Delayed contrast enhancement, a rim-like capsule surrounding the gland on contrast-enhanced sequences (the hypoattenuation halo), a non-dilated, ectatic pancreatic duct and the absence of peri-pancreatic fat hypoenhancement are other common features seen on CT scans (reviewed in ). A study using dual-phase CT to identify findings that aid in differentiating AIP from pancreatic adenocarcinoma showed that patients with AIP were more likely to have diffusely decreased enhancement of the pancreas, capsule-like rim, pancreatic strands, pancreatic calcifications, bile duct wall enhancement and renal involvement . MRI reveals enlargement of the pancreas with decreased signal intensity on T1-weighted images and increased signal intensity on T2-weighted images . The most common finding on EUS scans is diffuse or focal pancreatic enlargement along with a diffusely hypoechoic parenchyma .
A variety of serum markers have been used in the differentiation of AIP from pancreatic carcinoma. Serum IgG4 level is considered one of the most sensitive and specific markers for AIP. A serum IgG4 level greater than 140 mg/dL was reported to have a sensitivity, specificity and positive predictive value of 76%, 93% and 36%, respectively, and at levels greater than 280 mg/dL, these values are 53%, 99% and 75%, respectively . Our patient's IgG4 level was elevated at 385 mg/dL. The tumor marker CA 19-9 is another useful blood test in differentiating benign from malignant pancreatic disorders. The median sensitivity of CA 19-9 for detecting pancreatic adenocarcinoma is 79% (inter-quartile range (IQR), 70% to 90% and the median specificity is 82% (IQR, 68% to 91%) . However, levels of CA 19-9 may also rise in some benign pancreatic conditions, including obstructive jaundice. The CA 19-9 level usually decreases after decompression of the biliary system. Our patient's CA 19-9 level was initially 3282 U/mL and decreased to 129 U/mL after biliary decompression. Recently, an anti-PBP peptide antibody was detected in 33 (94%) of 35 patients with AIP, compared with 5 (< 5%) of 110 patients with pancreatic cancer . Other antibodies elevated in patients with AIP include anti-lactoferrin and anti-carbonic anhydrase II and anti-carbonic anhydrase IV, but these antibodies are available only for research purposes.
AIP has been shown to be responsive to corticosteroid therapy . The decision to treat patients with a corticosteroid is most often based on symptoms, imaging features consistent with AIP, an elevated IgG4 level and a low suspicion of cancer. A histological diagnosis of AIP is usually not required. An EUS-guided core biopsy or laparoscopic biopsy can be obtained to rule out pancreatic cancer. The typical corticosteroid dose is prednisone 30 mg/day to 40 mg/day for four weeks, followed by slow tapering by 5 mg/week. Patients should be followed closely for clinical, radiological and serologic resolution, which may be seen as early as two to three weeks after the commencement of therapy. Recurrence is seen in up to 40% of patients, and these patients may benefit from another course of corticosteroids or from the use of immunosuppressive agents such as azathioprine . In patients who do not respond to steroid therapy, the diagnosis of AIP should be re-evaluated.