Autoimmune pancreatitis (AIP) is an inflammatory disorder of pancreas. Two types have been identified: the diffuse and the focal or mass forming. Clinical presentation of AIP overlaps that of pancreatic cancer (PC). Sometimes serum IgG4 and CA 19-9 levels are unable to differentiate AIP from PC. Various series have shown that 5%–21% of resected pancreatic masses for suspected malignancy turned out to be AIP. Accurate diagnosis of focal AIP can avoid unnecessary surgeries. This paper elaborates the various imaging modalities useful in differentiating focal AIP from PC. 1. Introduction Autoimmune pancreatitis (AIP) is now a well-defined entity among the inflammatory diseases of the pancreas [1]. According to the Asian Diagnostic Criteria for Autoimmune pancreatitis [2], AIP has been classified into diffuse and focal types. Focal AIP is characterized by a segmental involvement of the parenchyma with the possibility of a low-density mass being present at imaging. Clinically, AIP patients and patients with pancreatic cancer (PC) share many features, such as preponderance of elderly males, frequent initial symptom of painless jaundice, development of new-onset diabetes mellitus, and elevated levels of serum tumor markers. Radiologically, focal swelling of the pancreas, the “double-duct sign,” representing strictures in both biliary and pancreatic ducts, and encasement of peripancreatic arteries and portal veins are sometimes detected in both AIP and PC. Several series indicate that in 5%–21% of resected pancreatic masses suspected of being cancerous, the final diagnosis was AIP [3–5]. Since AIP responds dramatically to steroid treatment, a correct diagnosis of the disease is important to avoid surgery. On the other hand, in the presence of a resectable pancreatic mass, the probability of cancer is very high (>90%). A misdiagnosis of AIP implies 2-3 weeks of steroid treatment and a one month delay in surgery, with the consequent risk of not operating because of the progression of the malignancy with the onset of metastasis or of vascular involvement. Korean Society of Gastroenterology proposed new diagnostic criteria to diagnose AIP (Table 1) which included imaging finding, serology, histopathology, and response to steroid treatment [6]. Table 1: Diagnostic criteria for autoimmune pancreatitis proposed by the Korean Society of Gastroenterology. There are some diagnostic difficulty in differentiating focal AIP and PC. Though serum IgG4 levels are elevated in AIP it was observed in a study by Tabata et al. [7], that only 77% of the 39 AIP cases had IgG4 levels >
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