PET CT Is Not a Single Test
Many people think of PET CT as one standard scan. In reality, PET CT is a platform that can use many different radioactive tracers, each designed to detect a specific biological process or target a specific type of cell. The choice of tracer determines what the scan can find. Using the wrong tracer for a given cancer is like using the wrong key for a lock. It simply will not work well.
The range of available PET CT tracers has expanded significantly in recent years. At specialized nuclear medicine centers, you may find a selection of tracers that cover a wide spectrum of cancers and clinical questions. Here is an overview of the major tracers and the situations where each one is most useful. You can explore our scans for a complete list of available imaging studies.
FDG: The Workhorse Tracer
FDG PET CT uses fluorodeoxyglucose, a radioactive sugar molecule. Cancer cells generally consume more glucose than normal cells because they are growing and dividing rapidly. FDG exploits this by accumulating in cells with high glucose metabolism.
FDG is the most widely used PET tracer and works well for many common cancers, including lung cancer, lymphoma, head and neck cancers, colorectal cancer, esophageal cancer, and melanoma. It is useful for initial staging, restaging, monitoring treatment response, and detecting recurrence.
However, FDG has limitations. Some cancers have low glucose metabolism and do not light up well on FDG scans. These include well-differentiated neuroendocrine tumors, prostate cancer, certain types of liver cancer, and some low-grade tumors. FDG can also accumulate in areas of infection or inflammation, which sometimes creates false positive results. For these situations, more specific tracers are needed.
PSMA: Prostate Cancer
PSMA PET CT targets prostate-specific membrane antigen, a protein that is highly overexpressed on prostate cancer cells. This tracer has transformed prostate cancer imaging. It can detect small metastatic deposits in lymph nodes, bones, and other organs that conventional imaging methods often miss.
PSMA PET CT is used for initial staging (especially in intermediate and high-risk prostate cancer), biochemical recurrence (when PSA levels rise after treatment), and treatment planning for radioligand therapy. It is far more sensitive and specific for prostate cancer than FDG PET CT or traditional bone scans.
FAPI: A Broad-Spectrum Emerging Tracer
FAPI PET scan targets fibroblast activation protein (FAP), which is found on cancer-associated fibroblasts in the tumor microenvironment. Unlike tracers that target the cancer cells themselves, FAPI targets the stromal cells that surround and support the tumor.
What makes FAPI interesting is its broad applicability. FAP is overexpressed in the stroma of many different cancer types, including pancreatic, gastric, breast, cholangiocarcinoma, and several others where FDG may not perform optimally. FAPI also has very low uptake in the brain and liver under normal conditions, making it useful for detecting lesions in these organs where FDG can have high background activity.
DOTANOC: Neuroendocrine Tumors
DOTANOC (and related tracers like DOTATATE and DOTATOC) PET CT targets somatostatin receptors, which are abundantly present on neuroendocrine tumors (NETs). These are a group of tumors that arise from hormone-producing cells found throughout the body, commonly in the gastrointestinal tract, pancreas, and lungs.
Neuroendocrine tumors are often slow-growing and have low FDG uptake, making FDG PET CT unreliable for this group. Somatostatin receptor-based PET CT is far more sensitive for detecting well-differentiated NETs and is also essential for selecting patients for peptide receptor radionuclide therapy (PRRT), a targeted nuclear medicine treatment.
CAIX: Clear Cell Renal Cell Carcinoma
CAIX PET CT targets carbonic anhydrase IX, a protein that is expressed in the vast majority of clear cell renal cell carcinomas (the most common subtype of kidney cancer). This tracer can help characterize kidney masses, detect metastatic disease, and potentially guide treatment decisions. It is particularly useful because kidney cancer is another tumor type where FDG has limited sensitivity.
CXCR4: Hematological Malignancies
CXCR4 PET CT targets the CXCR4 chemokine receptor, which is overexpressed in several blood cancers, including multiple myeloma, certain lymphomas, and Waldenstrom’s macroglobulinemia. This tracer provides information about the disease burden and distribution that can complement FDG PET CT findings, and it may help identify patients who could benefit from CXCR4-targeted therapies.
Exendin: Insulinomas and Beta Cell Imaging
Exendin PET CT uses a radiolabelled GLP-1 receptor agonist to detect insulinomas, which are rare insulin-producing tumors of the pancreas. These tumors are often very small and difficult to find on conventional imaging. Exendin PET CT has shown high sensitivity for localizing insulinomas, making it a valuable tool when CT, MRI, and endoscopic ultrasound fail to locate the tumor.
GPC3: Hepatocellular Carcinoma
GPC3 PET CT targets glypican-3, a protein overexpressed in hepatocellular carcinoma (primary liver cancer). This tracer can help characterize indeterminate liver lesions, stage HCC, and potentially monitor treatment response. It fills a gap left by FDG, which is often not sensitive enough for well-differentiated liver cancers.
Trivehexin: Integrin-Targeting
Trivehexin PET CT targets integrins (specifically alpha-v beta-3 and others), which are proteins involved in angiogenesis (new blood vessel formation) and tumor invasion. This tracer can provide information about tumor vascularity and aggressiveness across multiple cancer types. It represents a newer approach to imaging tumor biology rather than a specific cancer type.
How the Decision Is Made
The selection of the right PET CT tracer depends on several factors:
The type of cancer: Different cancers express different molecular targets. A prostate cancer patient needs PSMA, not FDG. A neuroendocrine tumor patient needs DOTANOC, not PSMA.
The clinical question: Is the scan for initial diagnosis, staging, treatment response assessment, or recurrence detection? The answer can influence tracer choice.
Previous imaging results: If FDG PET CT was negative but clinical suspicion remains high, a different tracer targeting the suspected tumor biology may be appropriate.
Treatment planning: Some tracers double as companion diagnostics for targeted therapies. For example, PSMA PET CT not only stages prostate cancer but also identifies candidates for PSMA-targeted radioligand therapy. Similarly, DOTANOC PET CT selects patients for PRRT.
Your Doctor Decides
Choosing the right PET CT scan is a medical decision that requires understanding the cancer type, its biology, the stage of disease, and the specific question that needs answering. This decision is made by your oncologist, nuclear medicine physician, or the multidisciplinary tumor board managing your care.
If you have been advised to undergo a PET CT scan, ask your doctor which tracer will be used and why it is the right choice for your situation. Understanding the purpose of the scan can help you feel more informed and prepared. A nuclear medicine specialist can explain the procedure, what to expect, and how the results will guide your treatment plan.