What is a PET/CT scan in simple terms?
A PET/CT scan (Positron Emission Tomography combined with Computed Tomography) is a scan that shows both:
- What your body looks like (structure)
- How your body is working (function)
It uses a small amount of radioactive sugar called FDG (fluorodeoxyglucose).
Cancer cells use more energy than normal cells, so they absorb more of this sugar and become visible on the scan.
What does a PET scan actually show?
A PET/CT scan shows how active tissues are inside your body. It uses a radioactive sugar to highlight areas of high energy use, which helps detect cancer, check if treatment is working, and identify if disease has spread.
Why do doctors use a PET scan instead of a normal CT or MRI?
CT and MRI scans show structure.
A PET scan shows activity.
Why this matters
A lump may:
- Look normal in size
- But still be active cancer
PET identifies this early.
The most important mistake patients make before a PET scan
Poor preparation is the number one reason for inaccurate results.
The biggest errors:
- Eating before the scan
- Exercising beforehand
- Poor diabetes control
Why this matters
PET scans rely on sugar metabolism.
If this is altered:
- Cancer may not be visible
- Or normal tissue may look abnormal
What cancers can a PET/CT scan detect (and what can it miss)?
PET scans are highly effective for:
- Lung cancer
- Lymphoma
- Bowel cancer
But can miss:
- Slow-growing cancers
- Low metabolic tumours
Important for your practice
Standard PET is not ideal for prostate cancer → PSMA PET is superior (high SEO opportunity).
When should you have a PET scan after chemotherapy?
- Standard: ~6 weeks after treatment ends
Why waiting matters
Chemotherapy causes:
- Inflammation
- Tissue repair
Scanning too early:
- Creates false positives
- Leads to unnecessary worry
How long after radiotherapy should you wait for a PET scan?
- Recommended: ~3 months
Why?
Radiotherapy effects last weeks to months.
These changes:
- Mimic cancer
- Reduce scan accuracy
Can you have a PET scan after surgery or a biopsy?
After biopsy
- Wait at least 7 days
After surgery
Depends on:
- Size of operation
- Area involved
Core principle
Inflammation = increased activity = possible false result
What medications can affect PET scan results?
Most medications are safe.
But three categories matter:
Can steroids affect PET scan accuracy?
Yes.
Steroids increase blood sugar.
This reduces cancer visibility on PET scans.
Do antibiotics interfere with PET scans?
Not directly.
But infections do.
Why?
Infection:
- Increases activity
- Looks similar to cancer
Should you stop medications before a PET scan?
Usually no.
Exceptions:
- Sugary medicines
- Specific clinical advice
What happens before a PET scan and how should you prepare?
You will be asked to:
- Fast for 4–6 hours
- Drink water
- Avoid exercise
- Keep warm
Why fasting before a PET scan is critical
PET scans depend on glucose.
If you eat:
- Blood sugar rises
- Cancer absorbs less tracer
Result: less accurate scan
Why exercise before a PET scan can hide cancer
Exercise activates muscles.
Active muscles absorb glucose.
Result:
- Muscles “light up”
- Cancer may be missed
Could you please explain what occurs during a PET scan?
- Injection of tracer
- Waiting period (~60 minutes)
- Scan (~25–30 minutes)
How long does a PET scan take in total?
- Total time: 2–3 hours
- Scan: 25–30 minutes
What does a PET scan feel like?
- Not painful
- Only small injection discomfort
- Requires lying still
What if you are claustrophobic?
Key facts:
- Scanner is short
- Head often outside
Options:
- Eye mask
- Eyes closed
- Sedative (if prescribed)
PET scan and diabetes: what you must do to avoid wrong results
Why diabetes matters
PET depends on glucose levels.
Poor control = inaccurate scan
Can you take insulin before a PET scan?
Yes—but carefully:
- Take insulin 4 hours before
- Then fast
Should you take diabetes tablets?
Usually:
- Skip before scan
- Bring them with you
Can infection or inflammation cause a false PET scan?
Yes—very commonly.
Causes of false positives:
- Infection
- Recent surgery
- Biopsy
- Exercise
Can you breastfeed after a PET scan?
You must pause temporarily.
Why?
The tracer passes into breast milk.
Typical advice:
- Express and discard
- Resume after a few hours
How accurate is a PET scan for cancer?
- Sensitivity: ~85–95%
Accuracy depends on:
- Cancer type
- Preparation
- Timing
What can cause a false positive PET scan?
- Infection
- Inflammation
- Recent treatment
- Physical activity
Who benefits most from a PET/CT scan?
Most useful for:
- Cancer staging
- Detecting recurrence
- Monitoring treatment response
FAQ: PET scan questions patients ask most
Do all cancers show up on a PET scan?
No. Some cancers are less active and may not absorb enough glucose.
Can a PET scan miss cancer?
Yes—especially:
- Small tumours
- Slow-growing cancers
Is PET better than MRI?
They answer different questions:
- MRI = structure
- PET = function
Why must I stay still?
Movement:
- Blurs images
- Reduces accuracy
PET/CT scan explained: how to get the most accurate result
A PET/CT scan is only as good as its preparation and timing.
The three things that matter most:
- Correct timing after treatment
- Proper preparation (fasting, no exercise)
- Good blood sugar control
If these are done correctly, PET scans:
- Improve diagnosis
- Guide treatment decisions
- Reduce uncertainty
