FDG-PET plays an expanding role in diagnosing several tumors including pancreatic cancers, as well as in identifying distant metastasis and recurrence. Moreover, it has been suggested that FDG-PET would be useful for differentiating pancreatic cancer from tumor-forming pancreatitis [4
]. Although the experience with FDG-PET in the evaluation of IPMNs is presently limited, recent studies have suggested that FDG-PET is more accurate than conventional imaging modalities in distinguishing benign from malignant lesions. Sperti et al. [5
] reported that FDG-PET had sensitivity, specificity, positive and negative predictive values and accuracy in detecting malignant IPMNs of 92, 97, 96, 95 and 95%, respectively, as compared to CT and/or magnetic resonance values of only 58, 82, 68, 74 and 72%, respectively, based on a prospective evaluation of 64 patients with suspected IPMNs. In this study, focal uptake with a SUV ≥2.5 was considered positive for malignancy. Hong et al. [6
] also demonstrated that FDG-PET/CT outperformed multidetector CT in detecting malignant IPMNs in a retrospective analysis of 31 patients with pancreatic IPMN. Moreover, other studies found FDG-PET to have additional value in the diagnosis of malignant IPMN. Tomimaru et al. [7
] reported that the combination of a mural nodule detected on CT and a SUVmax
of 2.5 obtained by FDG-PET offered the best diagnosis of malignant IPMN in a prospective investigation of 29 patients with histopathologically proven IPMN. Takanami et al. [8
] reported that FDG-PET/CT showed excellent diagnostic accuracy for differentiating between benign and malignant IPMNs with mural nodules on contrast-enhanced CT in a retrospective study of 16 patients with surgically proven IPMN. In our present cases, FDG-PET findings were useful as well for treatment determination because it was possible to confirm malignancy from the SUV (case 1: 3.4; case 2: 4.2). However, subtypes of IPMN were not discussed in earlier studies and little is known about differences in FDG-PET findings among subtypes of IPMNs.
Although IOPN has obvious malignant potential and preoperative diagnosis is important, clinical imaging features of IOPN are similar to those of other types of IPMN, and it is thus difficult to make a preoperative diagnosis of IOPN [9
]. FDG-PET findings of IOPN reportedly differed from those of IPMN [3
]. However, to the best of our knowledge, there have been only three case reports of FDG-PET findings of IOPN [3
summarizes the IOPN cases in whom FDG-PET was performed. Noji et al. [3
] reported the first case of IOPN showing strong accumulation of FDG in the tumor and suggested that IOPN may be differentiated from other IPMNs by FDG-PET based on the high metabolic activity of IOPN. As in the first case, high SUVs in the mural nodule within the cystic components were present in the other two cases. Thus, Kato et al. [9
] and Fischer et al. [10
] also described FDG-PET as being a potentially useful modality for distinguishing IOPN from other pancreatic cystic tumors. However, these three cases had extremely large tumors, approximately 10 cm in maximum diameter, with solid components. The mean lesion size was 6.4 cm (range 1.5–15 cm) in 20 patients with IOPN reported in the English and Japanese literature through 2008 [9
]. To confirm that IOPN has higher FDG uptake than other types of malignant IPMN, examination of FDG-PET findings of IOPN comparable in size to the average malignant IPMN is necessary. The reported average size of malignant IPMNs was 3.4 ± 1.8 cm in 445 surgical cases [11
]. The tumors in our cases were within the average size range of malignant IPMN. The average SUV of malignant IPMNs was reported to be 6.7 ± 3.6 (1 CIS and 14 invasive carcinomas) [6
], 4.7 ± 3.0 (3 CIS and 11 invasive carcinomas) [7
] or 2.7 ± 0.6 (8 CIS and 1 invasive carcinoma) [8
]. The SUVmax
of our cases were 3.4 (case 1) and 4.2 (case 2), not significantly different from that of malignant IPMN. Although more IOPN cases must be accumulated, the differentiation of IOPN from other malignant IPMNs by FDG-PET may be difficult in tumors of average size.
Summary of IOPN cases with FDG-PET findings
In conclusion, FDG-PET may be useful for diagnosing whether IOPN is a malignant tumor, as it is for IPMN. In IOPN of average size, differentiation between IOPN and malignant IPMN appears to be difficult, because the SUVmax in our cases did not show a ‘strong’ value and were similar to those of previously reported malignant IPMNs.