What Is a PET Scan?

A Positron Emission Tomography (PET) scan is a type of imaging test that shows how tissues and organs are working — not just what they look like. Unlike X-rays, CT, or MRI, which show structure, a PET scan shows function and activity.

It works by injecting a small amount of a radioactive tracer (commonly FDG, a sugar-based compound) into the bloodstream. Because cancer cells often use sugar at a much higher rate than normal cells, they absorb more of the tracer and appear as bright “hot spots” on the scan.

 

How PET Scans Are Used in Cancer

1. Detection & Diagnosis

• Helps identify cancer in tissues where tumors may not be visible on CT or MRI.
• Distinguishes between active cancer cells and scar tissue.

2. Staging

• Determines whether cancer has spread (metastasized) to other organs.
• Commonly used for lung, lymphoma, colorectal, head & neck, and esophageal cancers.

3. Treatment Planning

• Guides surgery or radiation by showing where the most active cancer is located.
• Combined PET/CT scans are standard, giving both structural and functional details.

4. Monitoring

• Checks if cancer treatment is working (tumors using less sugar = therapy effective).
• Detects recurrence earlier than some other imaging methods.

 

What to Expect During the Test

• The entire process takes 1.5–2 hours.
• After tracer injection, you rest quietly for 30–60 minutes while it spreads through the body.
• The scan itself usually takes 20–40 minutes.
• You lie still on a table that slides through a large circular scanner.

 

Benefits

• Shows biological activity of cancer, not just anatomy.
• Detects cancer spread with high accuracy.
• Useful in evaluating treatment response early.
• Combined PET/CT scans provide a complete picture.

 

Risks & Limitations

• Radiation exposure: From both the tracer and CT (if combined PET/CT).
• Limited resolution: Small tumors may not be detected.
• False positives: Infection or inflammation can also show as bright spots.
• False negatives: Slow-growing cancers (e.g., prostate, some breast cancers) may not absorb enough tracer.
• Not suitable for pregnant women or sometimes breastfeeding mothers.

 

Alternatives & Related Tests

• CT Scan: More detail on structure but not activity.
• MRI: Excellent for soft tissue anatomy.
• X-ray: Basic imaging, less detailed.
• Bone Scan: Sometimes used instead of PET for bone metastasis.

 

Questions to Ask Your Doctor

• Why is a PET scan recommended for me?
• Will I have a PET alone or PET/CT?
• How will this scan change my treatment plan?
• Are there risks with repeated PET scans?
• Do I need to stop any medications before the scan?

 

Patient Tips

• Fasting: Do not eat for 4–6 hours before the test; only drink water.
• Avoid strenuous exercise 24 hours before (muscles may absorb tracer and cause false results).
• Tell your doctor if you are pregnant, breastfeeding, or diabetic.
• Wear comfortable, metal-free clothing.

 

References

• National Cancer Institute (NCI): PET Scans in Cancer
• American Cancer Society: PET Scans
• Radiological Society of North America (RSNA): What is a PET Scan?