Somatostatin Receptor–Based Positron Emission Tomography Imaging
Both cross-sectional imaging and functional imaging with somatostatin receptor (SSTR)–based positron emission tomography (PET) can offer important information in the evaluation of carcinoid syndrome–associated neuroendocrine tumors (NETs).
Professor and Chief
Division of Medical Oncology
Department of Medicine
Member, UK Markey Cancer Center
University of Kentucky
“Whether patients have symptoms of carcinoid syndrome or a presentation that is more consistent with nonfunctional NETs, we can use SSTR-based DOTATATE PET/CT imaging for staging and to better understand the sites of metastatic disease.”
The presentation of NETs that have the potential to be associated with carcinoid syndrome may be quite varied, and a delay in diagnosis is common among patients with these tumors because the early symptoms are often vague and nonspecific. The first type of diagnostic imaging that should be done is typically a computed tomography (CT) scan. And, particularly in those whose advanced NETs are not functional, the tumors may be discovered incidentally, patients may have symptoms of abdominal pain or discomfort, or patients may have obstructive symptoms with weight loss or vomiting. When NETs are functional and patients are syndromic, common symptoms include diarrhea and flushing.
Whether patients have symptoms of carcinoid syndrome or a presentation that is more consistent with nonfunctional NETs, we can use SSTR-based DOTATATE PET/CT imaging for staging and to better understand the sites of metastatic disease. Combined DOTATATE PET and fluorine-18-fluorodeoxyglucose (18F-FDG) PET might be used in some scenarios, but the role of FDG PET is quite limited for the lower-grade small bowel NETs that typically cause carcinoid syndrome. Gallium-68 (68Ga)–DOTATATE PET has been the standard over the last decade, but copper-64 (64Cu)–DOTATATE PET is now available as well. Our experience up until recently was with using 68Ga-DOTATATE only, and we published some data with this technique using lean body mass–normalized standardized uptake value of the liver as an internal reference. 64Cu-DOTATATE PET has a much longer half-life, however, and is therefore more user friendly, so I believe that most centers will eventually prefer 64Cu-DOTATATE for these PET scans.
In addition to staging, DOTATATE PET is used in planning treatment for advanced NETs. If the patient does not have SSTR expression, we may not even want to use the depot formulation of SSTR agonists at all. For surgical planning, many of our patients have diffuse liver metastases that are not amenable to surgical resection, but there may be some abdominal peritoneal sites that can be removed surgically. DOTATATE PET/CT is excellent for picking up those peritoneal sites that are not easily visualized on the CT scan alone. And then, when considering peptide receptor radionuclide therapy with lutetium-177–DOTATATE, these PET scans are essential in qualifying patients for that treatment.
Still, DOTATATE PET is not required in all cases, as lesions are often well visualized on CT. PET may play a role in response assessment, but PET imaging is not used for routine surveillance. We will typically perform PET at the time of initial presentation, and the DOTATATE PET tends to be more of a one-time scan. If a patient undergoes debulking and a lot of their disease has been treated, we might want to get a new baseline PET scan because it might assist in the assessment of progression, especially when it identifies an area that is not seen on CT. Then, 6 years later, for example, we may get a DOTATATE PET again, as this can help with chronic management, but we would not use it for routine surveillance (eg, every 6 or 12 months) and would instead use standard cross-sectional imaging.
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