Two Tracers, Two Targets
PET CT scanning has become a cornerstone of cancer imaging, but not all PET scans are the same. The difference lies in the radioactive tracer used and what it targets in the body. The two tracers generating the most clinical discussion today are FDG and FAPI.
FDG (fluorodeoxyglucose) is a modified sugar molecule labelled with the radioactive isotope F-18. Cancer cells tend to consume glucose at a higher rate than normal cells, so FDG accumulates in metabolically active tumours. FDG PET CT has been the standard in oncological imaging for over two decades.
FAPI (fibroblast activation protein inhibitor) is a newer class of tracer, most commonly labelled with Ga-68 or F-18. Instead of targeting cancer cells directly, FAPI targets a protein called fibroblast activation protein (FAP) that is expressed on cancer-associated fibroblasts, the supportive cells in the tumour microenvironment. In many solid cancers, these fibroblasts are abundant, making FAPI a powerful imaging tool.
What Each Tracer Sees
FDG: Glucose Metabolism
FDG works because cancer cells are metabolically hungry. The more glucose a cell consumes, the brighter it appears on the scan. This principle holds true for most aggressive cancers: lung cancer, lymphoma, melanoma, colorectal cancer, and many others light up reliably on FDG PET CT.
However, FDG is not cancer-specific. Any cell with high metabolic activity will take up the tracer. This includes:
- Inflammatory tissue (post-surgical changes, infection, autoimmune conditions)
- Normal brain tissue (the brain is the largest consumer of glucose in the body)
- Heart muscle (particularly after eating)
- Active skeletal muscle
These sources of physiological and non-cancerous uptake can create false positives or obscure tumour signals, particularly in the brain and liver.
FAPI: Tumour Microenvironment
FAPI takes a fundamentally different approach. Instead of looking at how the cancer cell itself behaves, it images the stromal environment that surrounds the tumour. Cancer-associated fibroblasts express FAP in high concentrations, and FAPI tracers bind to this protein selectively.
The practical result is remarkably clean images. Because most normal tissues have low FAP expression, the background signal on FAPI PET CT is minimal. Tumours stand out with high contrast against surrounding structures.
Where FAPI Has Clear Advantages
Brain Imaging
This is one of the most striking differences between the two tracers. The normal brain consumes enormous amounts of glucose, creating a uniformly bright background on FDG PET CT. Detecting a brain tumour against this backdrop is like trying to spot a white object on a white table. Small lesions can be missed, and tumour boundaries are hard to define.
FAPI, by contrast, shows very little uptake in normal brain tissue. Brain tumours that express FAP appear as bright spots against a dark background, making them far easier to see and measure. For gliomas and brain metastases, FAPI PET CT offers a meaningful improvement in lesion visibility.
Liver Imaging
The liver processes glucose continuously, so FDG PET CT shows moderate background uptake in the liver. This can make it difficult to detect small liver metastases, particularly from cancers with moderate FDG avidity.
FAPI PET CT typically shows very low liver background, allowing liver metastases to be identified with greater confidence. For cancers like cholangiocarcinoma, hepatocellular carcinoma, and liver metastases from various primaries, FAPI can reveal lesions that FDG misses.
Cancers with Low FDG Uptake
Not all cancers are metabolically active enough to show up well on FDG PET CT. Mucinous tumours, certain gastric cancers, peritoneal carcinomatosis, and some low-grade malignancies may not take up FDG reliably. FAPI has shown the ability to detect several of these tumour types because it targets the stromal reaction rather than the tumour cell metabolism. Even if the cancer cells are metabolically quiet, the surrounding fibroblasts may still express FAP abundantly.
Peritoneal Disease
Peritoneal carcinomatosis, where cancer spreads across the lining of the abdominal cavity, is notoriously difficult to image. FDG PET CT can miss flat, sheet-like deposits along the peritoneum. FAPI has demonstrated superior sensitivity for peritoneal disease in several tumour types, including ovarian cancer, gastric cancer, and colorectal cancer.
Where FDG Still Leads
Despite FAPI’s advantages in certain areas, FDG PET CT remains the standard of care for many indications, and for good reason:
Lymphoma
FDG PET CT is deeply embedded in the staging, response assessment, and follow-up protocols for lymphoma. Decades of data support its use, and treatment guidelines (such as the Lugano classification) are built around FDG response criteria. FAPI uptake in lymphoma is variable and not yet standardised for clinical decision-making.
Response Assessment
Because FDG reflects metabolic activity, a decrease in FDG uptake after treatment reliably indicates that the cancer is responding. Response criteria like PERCIST are well-established and widely used. FAPI-based response criteria are still in development, and the relationship between FAP expression and treatment response is not yet as well understood.
Broad Clinical Validation
FDG has decades of clinical data across virtually every cancer type. Insurance coverage, clinical guidelines, and physician experience are all built around FDG PET CT. FAPI is still in the process of accumulating the evidence base needed for widespread adoption.
Can Both Scans Be Used Together?
Yes, and in some clinical scenarios, combining FDG and FAPI PET CT provides complementary information. FDG shows the metabolic aggressiveness of the tumour, while FAPI reveals the extent of the stromal reaction. Together, they can offer a more complete picture of the disease.
For example, in a patient with pancreatic cancer, FDG may show the primary tumour clearly, while FAPI may reveal peritoneal deposits or subtle liver metastases that FDG missed. This dual approach is not routine, but it is being explored in clinical practice and research settings.
A Quick Comparison
Target: FDG targets glucose metabolism in tumour cells. FAPI targets FAP on cancer-associated fibroblasts.
Background signal: FDG has significant uptake in brain, liver, and inflammatory tissue. FAPI has very low background in most organs.
Preparation: FDG requires fasting (usually 4-6 hours) and blood sugar control. FAPI requires no fasting and no blood sugar management.
Uptake time: FDG needs about 60 minutes after injection. FAPI can often be imaged within 60 minutes, with some protocols allowing earlier acquisition.
Clinical maturity: FDG has 25+ years of clinical data and established guidelines. FAPI is newer with rapidly growing but still limited evidence.
Which Scan Is Right for You?
The choice between FAPI PET CT and FDG PET CT depends on the type of cancer, the clinical question being asked, and what previous imaging has shown. In many cases, FDG remains the appropriate first choice. In others, particularly when FDG results are equivocal, when the cancer type is known to have low FDG avidity, or when brain or liver imaging is critical, FAPI may provide answers that FDG cannot.
This decision should be made collaboratively between your oncologist and nuclear medicine physician. They will consider your diagnosis, prior imaging findings, and what treatment decisions hinge on the scan results. Neither tracer is universally superior; each has strengths that suit different clinical situations.