PET-CT: risks, radiation and why to limit contact with people afterwards

What a PET-CT scan is

PET-CT is a hybrid imaging method that combines positron emission tomography (PET) and computed tomography (CT). Before the scan a radiotracer is injected into a vein, most often glucose labelled with fluorine-18 (FDG). It accumulates where metabolism is more active, above all in tumours and areas of inflammation. PET shows how tissues work, CT shows how they are built, and the fused image can reveal a tumour a few millimetres in size and show whether it is active.

Is PET-CT harmful: in brief

• The radiation dose of a PET-CT scan is usually 8–25 mSv. That is more than an ordinary CT of one area but far below the doses that cause radiation injury.
• The tracer decays quickly: its amount halves every 110 minutes, and after a day practically none is left in the body.
• A person who has had PET-CT is not a danger to others. The restrictions concern only close, prolonged contact with small children and pregnant women in the first 6–12 hours.
• Clothes, belongings and dishes do not become radioactive.
• Side effects are rare and are more often related to the CT contrast agent than to the radiotracer.

Radiation in PET-CT: what dose the patient receives

The exposure has two parts: from the radiotracer (about 5–7 mSv) and from the CT (from 2–3 mSv for low-dose CT to 10–15 mSv for diagnostic contrast CT). On modern scanners the total dose is closer to the lower end.

For comparison, in some parts of Ramsar in Iran the natural background reaches 260 mSv a year, and no rise in cancer rates has been found there. The body is able to repair the damage caused by low doses of radiation.

Which is more harmful, CT or PET-CT

In terms of dose, PET-CT is “heavier” than an ordinary CT of one area because it includes both a CT and a radiotracer. But comparing them by dose alone is wrong: a single PET-CT shows the whole body and often replaces several separate CT scans, and in a third of patients it finds changes that are not visible on CT or MRI. The doctor decides which method is needed for the clinical question.

Can PET-CT cause cancer or make a tumour grow faster

There is no evidence that PET-CT speeds up tumour growth. The theoretical additional lifetime risk of cancer from a dose of 10–25 mSv is estimated at hundredths to tenths of a percent. For a patient with a cancer diagnosis, the benefit of accurate staging and correctly chosen treatment is incomparably greater. This is exactly why PET-CT is not offered to healthy people “as a check-up”.

How often can PET-CT be done

There is no fixed limit: the scan is repeated when its result affects treatment, usually to assess response to therapy or when recurrence is suspected. The interval is set by the treating doctor.

Is a person dangerous to others after PET-CT

For a few hours after the scan a small amount of radiotracer remains in the body, and the person is a weak source of gamma radiation. An adult nearby receives a dose of hundredths of a millisievert, many times less than the annual natural background. So for adult family members, colleagues and fellow travellers a patient after PET-CT is not dangerous: you can use public transport, be in the same room and sit at the same table.

Why you should not be in contact with people after PET-CT

There is no complete ban on contact. The advice to limit contact applies to two groups: small children and pregnant women. The growing tissues of a child and a foetus are more sensitive to radiation, so reasonable caution applies: any dose that can be avoided is better not received.

In the first 6–12 hours after the scan (the clinic will give the exact time):

• do not hold small children in your arms or on your lap and do not sleep in the same bed with them;
• keep 1–2 metres away from pregnant women and do not stay close for long;
• brief contact, such as passing by, a hug or handing something over, does no harm;
• if you are breastfeeding, pause for 6–12 hours; during this time someone else can feed the baby with milk expressed in advance.

The next day all restrictions are lifted.

What to do with clothes after PET-CT

Nothing special. Radiation does not “settle” on things, so clothes, shoes, your phone and bag do not become radioactive and do not need to be thrown away or specially treated. The tracer leaves the body mainly in urine, so simple hygiene is enough on the first day:

• wash the clothes and underwear you wore for the scan as usual; if urine got on them, wash them separately;
• flush the toilet twice and wash your hands thoroughly; men are advised to sit down to urinate;
• empty your bladder more often.

How you feel after PET-CT and side effects

Most patients notice no change after PET-CT. Possible effects are:

• weakness and headache from several hours without food, which pass after eating;
• slight soreness or a bruise at the injection site;
• a feeling of warmth, a metallic taste or nausea if an iodine contrast agent was given for the CT;
• an allergic reaction to the radiotracer, which is extremely rare.

If you develop a rash, swelling of the face, difficulty breathing or severe weakness, see a doctor at once.

What to eat and drink after PET-CT

No special diet is needed: you can eat straight after the scan. Drink 1.5–2 litres of water during the day so that the tracer leaves the body faster. Patients with diabetes should return to their usual meals and medication as advised by their doctor.

Who should not have PET-CT

• Pregnant women: the scan is done only when it is vital and treatment cannot be chosen without it.
• People with high blood sugar: the result would be inaccurate, so glucose is stabilised first.
• Patients in a severe condition who cannot lie still for 20–30 minutes.

Claustrophobia is not a contraindication: the PET-CT tunnel is short and wide, and a sedative can be given if needed.

Myths about PET-CT

“PET-CT is a waste of money; CT or MRI is enough”

In studies, PET-CT revealed changes in a third of patients that were not visible on PET or CT alone. For example, in a patient with lung cancer CT showed only the primary tumour, while PET-CT found another lesion. Instead of an operation that would not have helped, the doctor prescribed radiotherapy.

“I will give off radiation for many days after the procedure”

The half-life of fluorine-18 is 110 minutes. After 12 hours about 1% of the injected tracer remains in the body, and after a day practically nothing.

“After PET-CT you have to throw away your clothes and live apart for a week”

No. It is enough to limit close contact with small children and pregnant women for 6–12 hours and follow ordinary hygiene.

When PET-CT is ordered

Most often the scan is needed when cancer has already been diagnosed, in order to:

• determine the stage, that is, how far the tumour has spread;
• choose treatment;
• assess the response to chemotherapy or radiotherapy, for example PET-CT in lymphoma;
• detect a recurrence before symptoms appear.

PET-CT is also used in cardiology, to assess the viability of heart muscle after a heart attack, and in neurology: for epilepsy, dementia and brain tumours. For prostate cancer, PET-CT with gallium-68 (PSMA) is used.

How to prepare for PET-CT

1. Avoid intense physical activity for 24 hours before the scan.
2. Have your last meal no later than 4–6 hours before the procedure; you can and should drink unsweetened water.
3. Do not chew gum or suck sweets, including cough drops.
4. Leave jewellery at home and wear comfortable, warm clothing without metal parts.
5. If you have diabetes, discuss your glucose-lowering medication and insulin with the doctor in advance.

Tell the doctor about chronic conditions, the medicines you take, allergies, pregnancy or breastfeeding.

How the scan is done

1. Tracer injection. The radiotracer is injected into a vein, after which you lie quietly in the warm for 45–60 minutes without talking or chewing, so that the tracer does not collect in muscle and fat.
2. Scanning. You lie on the scanner table while CT and PET images are taken over 20–30 minutes. It is painless.
3. Report. A radiologist fuses the images and describes the result.

PET-CT is an outpatient procedure: you can leave the clinic as soon as the scan is finished.

PET-CT abroad: where to have it and how much it costs

International patients have PET-CT at clinics in Turkey, Germany, Spain and Israel, without waiting lists and on modern scanners with a reduced radiation dose. For example, at Medipol University Hospital in Istanbul the scan is done on a Philips Gemini TF scanner with an open design that is comfortable for patients with claustrophobia.

The cost of PET-CT abroad starts from $650 and depends on the country, the clinic and the type of radiotracer. PET-CT with gallium-68 costs from $2,000.

Frequently asked questions

How soon after PET-CT can I be with children?

You can be in the same room straight away. It is better to wait 6–12 hours before holding a small child or sleeping next to one.

Can I use public transport after PET-CT?

Yes. For adult fellow passengers the dose is negligible. Try not to sit right next to pregnant women and small children.

Can I fly after PET-CT?

Yes. Bear in mind that airport radiation detectors may be triggered during the first day, so carry a letter from the clinic confirming the scan.

When will the result be ready?

Usually within 1–2 working days, and at clinics abroad often on the same day.

Need a PET-CT scan abroad? Send a request and Clinics Direct medical coordinators will choose a clinic, give you the current price and arrange the scan free of charge.