What Is FDG?
FDG stands for fluorodeoxyglucose, a modified form of glucose (sugar) that has been labelled with a small amount of radioactive material. When injected into the body, FDG travels through the bloodstream and is taken up by cells that are actively using glucose for energy.
Cancer cells typically consume glucose at a higher rate than normal cells. This increased uptake makes them visible on a PET scan as bright spots, or areas of high metabolic activity. By combining this metabolic information with the anatomical detail from a CT scan, an FDG PET CT provides a complete picture of disease in the body.
Common Reasons for an FDG PET CT
Cancer staging: FDG PET CT is widely used for staging many types of cancer, including lung cancer, lymphoma, colorectal cancer, head and neck cancers, melanoma, and oesophageal cancer. Staging determines how far the cancer has spread, which directly affects treatment planning.
Treatment response: Oncologists often order FDG PET CT scans during or after treatment to assess whether the cancer is responding. A decrease in FDG uptake suggests the treatment is working, while persistent or increased uptake may prompt a change in approach.
Recurrence surveillance: After completing treatment, if there are clinical signs or blood markers suggesting the cancer may have returned, an FDG PET CT can help identify the location and extent of recurrence.
Characterising a lesion: When another imaging test has found a mass or abnormality, an FDG PET CT can help determine whether it is likely malignant or benign, though a biopsy is often still needed for confirmation.
Which Cancers Does FDG PET CT Detect Well?
FDG PET CT is effective for cancers that are metabolically active and consume glucose rapidly. It performs particularly well for lung cancer, lymphoma (both Hodgkin and non-Hodgkin), melanoma, head and neck cancers, and oesophageal cancer. It is also commonly used for colorectal cancer, breast cancer, and cervical cancer.
Limitations of FDG PET CT
Not all cancers show up clearly on FDG PET CT. Some tumours grow slowly and do not consume much glucose, making them difficult to detect with this tracer. Prostate cancer is a notable example, as many prostate cancers have low FDG uptake. For prostate cancer, a PSMA PET CT is generally more informative.
Similarly, some cancers in the brain can be challenging to evaluate with FDG because the brain itself has naturally high glucose metabolism, making it harder to distinguish tumour uptake from normal brain activity.
FDG uptake is also not exclusive to cancer. Inflammation, infection, and even recent surgery can cause areas of increased glucose metabolism that appear similar to cancer on a PET scan. This is why the nuclear medicine physician interprets the images in the context of your full medical history, and additional tests may be needed in some cases.
Preparation and the Scan Day
Preparation for an FDG PET CT typically involves fasting for 4 to 6 hours before the scan. This is because eating raises blood sugar levels, which can interfere with the tracer uptake. Patients with diabetes should inform the team in advance, as special instructions may apply.
You should also avoid strenuous exercise for 24 hours before the scan, as active muscles take up more FDG and can create confusing signals on the images. On the day of the scan, you will receive the FDG injection, rest quietly for about an hour, and then undergo the scan, which takes approximately 20 to 30 minutes.
After the Scan
The images are reviewed by a nuclear medicine specialist who prepares a detailed report for your referring doctor. FDG leaves the body naturally through urine within several hours. Staying hydrated after the scan helps this process.
Your doctor will discuss the results with you and explain how they fit into your overall treatment plan. An FDG PET CT result is one piece of the puzzle, and treatment decisions are always made in the context of the full clinical picture.