I.
Conventional:Computed tomography CT
CT scans are usually the initial imaging study.
PROS:
Would be anatomical useful modality for disease staging and surgical planning.
Primary NETs and their metastases are generally hyper-enhancing,
best seen in the arterial phase of a triple phase CT scan
Useful for operative planning,
disease staging
CONS:
Not a functional study; weak in detection of the primary in 95% of cases
High radiation exposure
Optimal imaging of NETs requires IV contrast
Magnetic resonance imaging
PROS:
MRI is the best to detail hepatic metastases in NETs
Would be useful good for the primary pancreatic NETs.
Good pancreatic resolution
No radiation exposure
Gadolinium contrast is safer if renal dysfunction
CONS:
not useful in the detection of primary small bowel lesions or their associated lymphadenopathy.
Poor assessment of GI tract,
mesenteric lesions
Cannot image patients with metal implants
Not a functional study
18FDG positron emission scanning
PROS :
Good for high grade NETs
Good anatomic resolution
CONS:
Unfortunately,
Most NETs tend to be metabolically inactive and fail to take up the tracer
Somatostatin receptor-based imaging techniques
- Somatostatin is secreted by neuroendocrine cells,
activated immune cells and inflammatory cells.
- It affects its antiproliferative and antisecretory functions by binding to one of five types of somatostatin receptors (SSTRs) being highly expressed on well-differentiated NETs,
and their ubiquity on NET cell surfaces makes them ideal targets for treatment and imaging.
Octreotide analog studies
-
111In-DTPA-D-Phe-1-octreotide is the ligand used by the modern OctreoScan.
This tracer was easier to prepare,
cleared primarily by the kidneys ; solving the problem of accumulation within the GI tract of the old one 123I tracer,
and had a much longer half-life (optimal image acquisition occurs at 24 h post-injection of the radiotracer).
Due to OctreoScan being a functional (i.e.,
SSTR-specific) full body scan,
it can often identify lesions that were missed by CT or MRI.
The OctreoScan is a useful,
especially when fused with SPECT and CT.
PROS:
- Functional study
- Level of uptake can be graded
- Provides whole body image (on one shot single study)
- Predicts response to therapy
CONS:
- High background in Gl tract,
which may obscure midgut NETs
- Patient must be scanned twice in 24 h
- Low anatomic detail
- Need to hold Octreotide prior to scan
DOTA-PET
The newest advance in somatostatin receptor-based imaging is the introduction of 68Ga-labeled radioligands,
whose uptake is measured by PET scan.
68Ga-DOTATOC -PET VS OctreoScan
· easier and cheaper to synthesize than standard octreotide-analog based ligands,
· scanning 1 h postinjection vs OctreoScan,
(which requires patients to be scanned at 4 and 24 h postinjection),
· and allow for quantifying the uptake of the ligand within the lesions.
· superior in spatial resolution
· 68Ga-PET studies are fused with CT to improve anatomic localization.
· universally demonstrated the superiority of 68Ga-PET in detection of NET primary tumors and metastases.
· 68Ga-DOTA PET imaging can be used to determine which patients might benefit from Octreotide and PRRT.
PROS
· Functional study
· Uptake can be quantified
· Good anatomic resolution
· Provides whole body image
· Image acquired over 2 h
· Predicts response to therapy
CONS
· Physiologic uptake in uncinate process,
pituitary,
spleen and kidneys may be confused for tumor uptake
· Available at a limited number of centers
Radioiodinated (123I) metaiodobenzylguanidine (MIBG):
- used to image catecholamine-secreting NETs such as pheochromocytomas & paragangliomas used to confirm their diagnoses and define the extent of metastatic disease
PROS
- Functional study
- High specificity for pheochromocytoma,
paraganglioma,
and glomus tumors
- Provides whole body image
- Predicts response to therapy
CONS:
- High background
- Low anatomic detail
- Many medications can interfere with the scan and must therefore be held
- Thyroid must be blocked prior to scan
- Patient scanned 24 h after contrast administration