Some studies reported modest improvement in the performance of FDG PET compared with CECT in patients with pancreatic masses, with sensitivity and specificity ranging from 30% to 49% and 63% to 93%, respectively, for evaluation of lymph nodes. Lesser tumor volume in affected lymph nodes and strong photon scatter from the primary tumor (penumbra effect) may partially explain the poor performance of FDG PET for lymph node staging [16, 17]. Metabolic information of FDG PET may complement that of CT for nodal staging, because even low-grade metabolic activity on fused imaging in a rounded centimeter-or-more-sized node may be indicative of nodal metastases.

FDG uptake in hepatic lesions strongly favors metastatic involvement; also the absence of FDG uptake in suspicious liver lesions seen on conventional imaging rules out metastases [18]. This dilemma arises mostly in case of solitary focal liver lesion that appears suspicious on triphasic CT/ultrasonography; here FDG uptake is a clincher as the presence of uptake favors malignancy (Fig. 7.1d, e, arrows) and further can be managed by liver-specific interventions.

In a study comparing the performance of hepatobiliary contrast-enhanced MR imaging and FDG PET, MR imaging was more accurate in depicting small liver metastases, with a reported accuracy of 97.1% compared with 85.3% for FDG PET [19].

Small volume disease in the form of nodular peritoneal implants often seen as stranding and rarely as frank “caking” pose a diagnostic challenge on CT, with reported sensitivity of 65–88% and specificity of 38–63% [20]. Peritoneal implants were found at staging laparoscopy in 7% of patients with locally unresectable pancreatic cancer and no evidence of metastasis at CECT [21].

Since PET/CT imaging is a whole body study, it is the best modality for detecting distant metastases at any site.

Most of the centers incorporate breath-hold plain CT of chest in PET/CT protocol, for detection of metastatic lung nodules. This is because detection of subcentimeter-sized nodules is beyond the resolution of even modern-day PET scanners, and hence plain thin-section breath-hold CT serves the purpose.

Thus, PET/CT impacts management change in patients deemed “operable” on conventional imaging, by detecting distant metastases, thus avoiding the cost of futile surgery.

PACs are naturally aggressive cancers, and following the natural history, after surgery, 72–92% of pancreatic adenocarcinomas recur locally within 2 years [22]. Locally recurrent tumors are usually not resectable; however, radiation therapy or local ablation either with radiofrequency or cryoablation may be a palliative option. Postoperative changes in the surgical bed and early tumor recurrence have overlapping morphologic characteristics, as a result, differentiating between them is difficult on CECT. Moreover, it is often difficult to obtain an adequate tissue sample because desmoplastic reaction is known to be associated with pancreatic cancers. The use of FDG PET to depict tumor recurrence is promising, particularly when CT findings are equivocal [23, 24]. Increased FDG uptake in the surgical site at 3 months following surgery is usually indicative of recurrence (Fig. 7.2). The reported sensitivity of FDG PET for depicting tumor recurrence is 96% compared with 39% for CT and MR imaging [23]. Moreover, after resection, tumor relapse is depicted at FDG PET earlier than it is at CT, with higher sensitivity (98%) and specificity (90%) [24].

PET combined with CECT plays a role in monitoring response to chemo- and radiation therapy in patients with unresectable pancreatic cancer [25, 26]. Significant reduction in FDG uptake may precede volumetric reduction at CT and may be proportional to the change in tumor size at subsequent follow-up examinations (Fig. 7.3). Therefore, earlier depiction of tumor response to therapy at FDG PET could influence the continuation or withdrawal of treatment [27]. Moreover, some recently published studies reported that FDG PET/CT might have prognostic value because tumors with a higher baseline SUV_max are more likely to recur in the early post-operative period. SUV_max is also an independent predictor for overall survival in patients with locally advanced pancreatic cancer [28, 29]. Postoperative inflammatory changes in the pancreas, radiation therapy, or stent placement may also cause some FDG uptake. To minimize these false-positive results, it is recommended that follow-up PET or PET/CT be performed at least 6 weeks after surgery [30].