PET CT helps radiation oncologists see which parts of a tumour are most active, so radiotherapy can target them accurately. Learn how it is used and how the scan is done.
PET CT for radiotherapy planning is a special scan done in the same position as your treatment. It shows the active tumour and involved lymph nodes, so the radiation oncologist can outline the target more accurately, protect healthy tissue and avoid missing disease. It is used mainly in lung, head and neck, oesophageal, cervical and lymphoma treatment planning.
Why Planning Needs Accurate Targets
Radiotherapy works best when the beam covers the whole tumour and as little healthy tissue as possible. CT shows shape, but cannot always tell tumour from scar, fluid or collapsed lung. PET shows which part is biologically active. Fusing the two gives a more reliable target. See what a PET CT scan is.
CancerHow PET CT helps radiotherapyLung cancerSeparates tumour from collapsed lung; finds active nodesHead and neck cancerShows primary and neck nodes; guides doseOesophageal and cervical cancerFinds nodes beyond the main tumourLymphomaTargets residual active diseaseBrain tumoursSpecial tracers help in selected casesHow a Planning PET CT Is Different
- You lie on a flat table top like the treatment couch.
- Masks, head rests or body supports are used so the same position can be repeated during treatment.
- Tattoo marks or reference points may be made on your skin.
- The scan is usually done within a few weeks of starting radiotherapy.
The images are sent to the planning system, where the oncologist outlines the target.
What Changes in the Plan
PET can show nodes that look normal on CT, which may extend the radiation field, or show that a large "mass" is partly scar, which reduces the target size. In some patients, it also finds distant spread, which changes treatment from curative radiotherapy to systemic therapy. See PET CT for cancer detection and staging.
Preparation
Fasting for 4 to 6 hours and controlled blood sugar still apply. You will rest quietly after the injection. Bring your biopsy report, earlier scans and treatment plan. Tell the team about claustrophobia if a mask is needed, since a trial fitting can help. See PET CT preparation.
Limits
Inflammation, infection and recent surgery can look active and enlarge the target. Very small tumours may be missed. The scan shows activity at one time, so the oncologist may adjust the plan during treatment if the tumour shrinks. MRI is added for brain, spine and pelvic tumours. See PET CT vs MRI.
A Practical Example
Ramesh, 63, has lung cancer with a part of the lung collapsed behind the tumour. On CT alone, the tumour edge is unclear. A planning PET CT shows the active tumour clearly and one active node in the chest. His oncologist outlines a precise target, spares more healthy lung and plans combined chemoradiation.
After the Scan and Next Steps
After the scan, the physics and planning team prepares the radiation plan, which can take several days. Treatment then starts, usually for several weeks. Follow skin care advice, eat well, drink water and report breathlessness, swallowing difficulty or severe tiredness. A follow-up PET CT after treatment checks response.
Questions to Ask Your Doctor
- Will PET CT change the area that is treated?
- Do I need a mask, and can I practise fitting it?
- How soon after the scan will radiotherapy begin?
- When will my response be checked?
Common Mistakes to Avoid
- Eating or taking sweet drinks before the scan.
- Moving out of position during the scan.
- Skipping skin and nutrition advice during radiotherapy.
Booking, Timing and Report
Ask your doctor for the best timing of the scan. Book a morning slot so that fasting is easier, arrange someone to accompany you, and allow 2 to 3 hours at the centre. Carry biopsy, scan and blood reports and a medicine list. Reports are usually ready within 24 to 48 hours, and images can be given on a CD or digital link for your doctor.
Frequently Asked Questions
What is a radiotherapy planning PET CT?
A PET CT done in treatment position to outline the tumour.
Is it different from a regular PET CT?
Yes, the position and setup are different.
Does it hurt?
No.
Do I need fasting?
Yes, 4 to 6 hours.
Will it change my treatment?
It can change the field or the plan.
Is a mask uncomfortable?
Many people find it tight but tolerable.
How long does it take?
About 2 to 3 hours in total.
Can inflammation affect planning?
Yes, so the doctor reads it carefully.
When is the report ready?
Usually 24 to 48 hours.
Book Your Test at Ganesh Diagnostic
Ganesh Diagnostic & Imaging Centre in Rohini, Delhi offers this test with trained radiology staff and clear reports. You can book an appointment online, check our branches across Delhi NCR, contact us for current rates, or explore all our services. Reports can be downloaded online through our patient portal, so you do not need to make a second trip just to collect them.
This article is for general information only. It does not replace advice from your doctor. Always follow your doctor's instructions for your test.
