The presence of extrapancreatic lesions, in other words, other organ involvement (OOI), is categorized as one of the cardinal features of type 1 AIP in the International Consensus Diagnostic Criteria for Autoimmune Pancreatitis (ICDC) [2, 3, 10, 11]. OOI is also part of the manifestation of IgG4-related systemic disease. OOI may be diagnosed by histological evaluation of tissue, by imaging (proximal bile duct stricture, retroperitoneal fibrosis), or by clinical examination (salivary gland enlargement). Concomitant extrapancreatic FDG uptake by other organs, such as the salivary glands and bile duct, has been shown to be indicative of systemic disease [5, 7, 8]. Lee et al. reported that abnormal FDG uptake by extrapancreatic lesions was found in 12 of the 17 patients (70 %). The salivary gland was the most frequent site of FDG uptake, followed by the retroperitoneal space, kidneys, thyroid, hilar lymph nodes, bile ducts, and lungs [5]. The diagnostic impact of PET for OOI was evaluated in some studies. Ozaki et al. compared PET findings between AIP and pancreatic cancer [9]. Hilar lymph node involvement was significantly more frequent in patients with AIP than in those with pancreatic cancer; however, there was no significant difference in lacrimal gland, salivary gland, biliary duct, and retroperitoneal space involvement. The prevalence of OOI differs across studies, because PET/CT findings in patients with AIP have been explored in only a small number of studies. In any case, extrapancreatic FDG uptake concomitant with pancreatic uptake may support a diagnosis of AIP (Fig. 13.2).

In some cases, it is difficult to distinguish AIP from pancreatic cancer even after a negative workup for pancreatic cancer. In cases of AIP, steroids would result in a definitive improvement of imaging abnormalities, including biliary strictures and pancreatic enlargement. Steroids are indicated under the following circumstances: (1) patients with typical imaging findings who have appropriate collateral findings of AIP or (2) patients without typical imaging findings who have a negative workup for cancer and have appropriate collateral evidence of AIP [12]. The recommended duration of steroid therapy is 2 weeks based on the study by Moon et al., the only study which has examined this issue [2, 13]. Response to steroid therapy was assessed using morphologic imaging modalities such as CT, ERP, and/or MRCP in their study. FDG-PET is a functional imaging modality that reflects the severity of inflammation and regression of inflammation after steroid therapy. We evaluated changes in FDG uptake before and after steroid therapy in 6 patients with AIP and in 3 patients with pancreatic cancer [14]. Changes in FDG uptake in pancreatic and extrapancreatic lesions were estimated semiquantitatively and visually within 1 week. FDG uptake was dramatically decreased within a week in most cases of AIP (5/6) (Table 13.2 and Fig. 13.3), whereas FDG uptake was unchanged or increased in cases of pancreatic cancer (Fig. 13.4). Our study indicates that FDG-PET may allow a more rapid (within 1 week) and objective assessment of the response to steroids, as compared with morphological imaging modalities.