PET scans Q&As

My daughter is going for a PET scan. Can you tell me about this?

The initials PET stand for positron emission tomography.

The test involves having an injection of a small amount of radioactive material. Using the signals from this radioactive injection a scanning machine can build up a picture of the part of the body. Like other sorts of scans PET scans can be used to look for abnormalities in the tissues, in particular whether there might be a cancer. Most scans (such as CT , MRI or ultrasound scans) will show an abnormality as a shadow on the scan but can not always tell what that abnormality might be. With PET scans however, if there is an abnormality it usually means that it is due to either cancer or to inflammation. So PET scans may sometimes provide information that is not available from other scans. Depending on the sort of radioactive material given in the injection PET scans can also give information about blood flow or the uptake of drugs in the tissues.

Usually people are asked not to eat or drink for six hours before the scan. The radioactive injection is given into a vein in the arm one hour before the scan itself. The injection is quite small and is made up of a glucose (sugar) solution containing the tiny dose of radioactive material. It is important to be as relaxed as possible for the scan itself so after the injection people are usually asked to lie or sit in a darkened room whilst they wait for the examination. Sometimes a small dose of diazepam (Valium) is given, as a tablet, one to two hours before the scan to help with relaxation.

The scan itself involves lying on a couch which moves through a large ring shaped scanner.This contains the sensors for the signals from the radioactive material. Depending on the area which is being scanned the examination can last anywhere from three quarters of an hour to two hours. Some people worry that they might feel claustrophobic during the scan but this is very rare.

Apart from the small injection before the scan the whole procedure is quite painless. There are no side-effects from the scan but if people are given diazepam as part of the preparation before the examination then they may feel a little drowsy for a few hours.

Initially PET scans were used for studying the brain but they are now being used to look at most other parts of the body.

PET scans are not a routine test and are usually only used for the small number of patients for whom other types of scans cannot give all the information their doctors need.

At the present time PET scans are only available in a very few hospitals so having a PET scan may well involve travelling some distance. However, there are plans to open more PET scanners across the UK in the next few years.

My daughter is pregnant and she has just had cancer diagnosed. If she needs any x-rays and scans will she still be able to have these even though she is pregnant?

Cancer is diagnosed during about 1 in every 1000 pregnancies. This means that the combination of cancer and pregnancy is uncommon. However, when it does occur it is a situation which raises many questions.

The benefits to the mother of any scans and x-rays must be balanced against the possibility of any harm to her unborn child. Another issue is that because cancer during pregnancy is so unusual there is relatively little information available and this can lead to uncertainty over the best way to handle the problem.

So the pattern of care for anyone who is found to have cancer during her pregnancy is likely to be worked out on an individual basis. The doctors will take account of such things as

• the type of cancer
• how big it is
• whether it has spread
• the stage of the pregnancy
• the different types of treatment that could be used
• and the feelings and wishes of the mother, her partner and their relatives.

Having said this, it is possible to make some general comments about the use of scans and x-rays during pregnancy. But before this it might be helpful to describe the various stages in the progress of a normal pregnancy. These are:

• preimplantation: this is the time from when fertilisation of the female egg (ovum) takes place until the fertilised egg becomes firmly attached to the wall of the womb (the uterus). This usually takes about 8 days.
• embryonic stage: this is the period during which the embryo develops and all the major organs of the body are formed (so it is also known as the period of organogenesis). This usually lasts from the 8th day to about the 8 the week of the pregnancy).
• foetal stage: during this time, the various organs that have been formed grow in size and become increasingly mature. Although most of the body's organs will have been formed during the embryonic phase a few important tissues are still developing during the foetal period, these include the nerves, the lung tissue and the bone marrow. The foetal stage lasts from the 8th week until the time of birth, at about 38 weeks.
  

A normal pregnancy lasts about 38 weeks and another way that is used to describe its progress is to talk about three 'trimesters', each trimester being about twelve weeks. So the first trimester covers the period of preimplantation, the development of the embryo and the beginning of foetal growth. The second trimester covers the early and middle stage of foetal growth and third trimester covers the final three months, til birth.

Any damage to the unborn child can lead to a number of different outcomes. These include:

• spontaneous abortion during the early part of the pregnancy
• a child born with congenital malformations or birth defects
• premature birth with the child being delivered early
• a low birth weight
• problems which appear during childhood or adolescence, such as delayed growth, difficulties with learning and infertility
• an increased risk of cancer development in later life.
  

This list looks alarming but it is important to remember that many of these problems occur with 'normal' pregnancies when there are no other illnesses or treatments involved. For example, it is estimated that between 1 in 8 to 1 in 10 pregnancies end in a miscarriage or spontaneous abortion, that about 1 in 50 babies are born with obvious birth defects or congenital abnormalities and about 1 in 10 have very minor, less obvious abnormalities.

Tests for someone with cancer are used to check on the size of the growth and whether or not is has spread. They may include: ordinary x-rays (like a chest x-ray), ultrasound scans, CT scans (also known as CAT scans or computerised scans), magnetic resonance imaging (MRI scans), radio-isotope scans (like bone scans) and positron emission tomography (PET scans).

The main concern over tests in pregnancy is whether they involve the use of ionising radiation. In high doses, radiation can be damaging to the unborn child, particularly during the first few weeks of pregnancy.

X-rays, CT scans, radio-isotope scans and PET scans involve the use of ionising radiation. The doses of radiation involved are small so the risk of damage to the unborn child is likely to be small. However, doctors try to avoid using these tests during pregnancy, especially during the first trimester. The one exception is a chest x-ray with protective shielding to the pelvic region. The dose to the foetus from this is so tiny that it is considered to be a 'safe' test.

Ultrasound scans and MRI scans do not use ionising radiation and there is no recorded evidence that they can cause any harm to the embryo or the foetus. So usually any 'imaging' tests that need to be done for a woman who is pregnant and who has cancer can be safely and effectively done using a combination of a chest x-ray and appropriate ultrasound and MRI scans.

Sentinel node biopsy testing is sometimes used in breast cancer or malignant melanoma to discover whether the cancer has spread to nearby lymph nodes. During this test a very small dose of radioactive material may be used. Specialists hold different views on whether this test is safe for use in pregnancy. Some feel that as the dose is so tiny the risk to the unborn child is negligible and the test is safe to use. Other doctors feel that this test should be avoided in pregnant women.

For each woman the doctors will work towards a safe and effective, pattern of tests, to get the information needed to plan her treatment. Obviously the woman and her family should be involved in discussions about what choices there are. In this way the final plan of action will offer the best possible chances of success for the mother with the least possible risk of harm to her unborn child.