The Supporting Role of 18FDG-PET in Patients with Neuroendocrine Tumors

Abstract

There has been a marked, unexplained increasing incidence of neuroendocrine tumors (NETs) over the past few decades. This may be due to a combination of different factors, such as changes in the environment with new exposures to injurious agents or an increased awareness of these neoplasms, but perhaps most of all, improvements in and utilization of body imaging. It has been estimated that as many as 72 million computed tomographic (CT) scans are done per year in the United States, a threefold increase since 1993.1 Abdominal CTs will detect liver metastases, and metastatic disease is found in 30 % of patients with small bowel NETs and 64 % of those with pancreatic NETs.2 Primary pancreatic NETs are commonly seen on CT if they are greater than 1 cm in size and appropriate administration of intravenous contrast is given. Primary small bowel NETs are more subtle because they are frequently small and intramural. They may show up as thickening of the bowel wall or a mass, but more commonly, one may see enlarged, calcified nodes in the mesentery.Although anatomic imaging (CT, MRI) has been very useful, the finding of a mass in the pancreas, mesentery, or liver does not define a tumor as being neuroendocrine in origin. This is where functional imaging for NETs can be useful. For these studies, octreotide is conjugated to a radioisotope, which accumulates in areas with a high density of somatostatin type 2 (SST2) receptors, which are expressed on the cell membranes of the majority of NETs. This has evolved into the exam we know today as OctreoScan, where patients are given an intravenous injection of 111 In-octreotide, and then scintigraphy is performed at 4 and 24 h. One important early study optimistically quoted finding uptake in 96 % of small bowel and 68 % of pancreatic NETs, but never specified whether the uptake seen was in primary tumors or their metastases.3 Nevertheless, this imaging modality has turned out to be useful in NETs from a multitude of primary sites, as long as they express SST2 receptors. Improvements have come over time with the administration of a bowel preparation before imaging, combining with single-photon emission computed tomography (SPECT), and more recently, by coregistration with CT scans. One recent study evaluating OctreoScans in patients with surgically confirmed small bowel NETs found that either the primary tumor or adjacent nodes had uptake in 74 % of cases, and liver metastases in 66 % (of cases where metastases were present).

The Surgical Management of Small Bowel Neuroendocrine Tumors

“…Therefore, 18 FDG-PET/CT may have value for staging, prognosis, and selecting NET patients who might benefit from medical versus surgical therapy, but the utility of these scans appears to be limited to patients with higher grade tumors. 130 …”

Comparison of the Impact of 68Ga-DOTATATE and 18F-FDG PET/CT on Clinical Management in Patients with Neuroendocrine Tumors

“…18 F-FDG PET/CT imaging has also been compared with 68 Ga-DOTA peptide imaging in several studies, which have shown it to have variable sensitivity in detecting NETs (15)(16)(17)(18)(19)(20)(21)(22). However, the presence of increased glucose in NETs highlights an increased propensity for invasion and metastasis, and 18 F-FDG PET/CT accordingly has higher sensitivity in delineating disease extent, especially in aggressive and high-grade tumors (23). Detection of a higher number of lesions is nevertheless not always followed by a change in disease stage and, most importantly, does not always affect the therapeutic approach.…”

Detection of Bone Metastases by 68Ga‐DOTA‐SSAs and 18F‐FDG PET/CT: A Two‐Center Head‐to‐Head Study of Gastroenteropancreatic Neuroendocrine Neoplasms

“…In this study, we observed a significant correlation between dual-tracer PET/CT imaging and tumor differentiation. 18 F-FDG performs well for the evaluation and management of high-grade NENs (poorly differentiated and aggressive) [30], but has limitations for the determination of metastatic lesions in well-differentiated NENs, because 18 F-FDG PET/CT is positively correlated with the degree of tumor malignancy and is less sensitive in well-differentiated NETs. There is thus a significant positive correlation between the SUVmax of BMs detected by 18 F-FDG and Ki-67 index [31].…”