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Thursday, May 22, 2008

CT scan Q&As

I am 55 years old and feel very well. I recently read in the newspaper about a low dose CT scan which can scan the whole body and can pick up cancers at an early stage. Would it be a good idea for me to have one of these

A CT scanner uses x-rays which are fed into a computer to produce a detailed picture of the inside of the body.

CT scans are widely used to help diagnose cancer and also to find out the size of tumours and whether they have spread. Despite this CT scans are not routinely used for screening (that is, the early detection of cancers which are causing no symptoms, in people who are apparently quite well). There are a number of reasons for this, including:

  • although CT scans are becoming more sensitive they still cannot detect very small cancers and so a 'negative' scan does not guarantee that there is no cancer there
  • CT scans are better at detecting cancer in some tissues than in others which means that in some parts of the body even quite large tumours could still be 'missed' by a whole body scan
  • CT scans often can't tell the difference between benign (non-cancerous) and malignant (cancerous) tumours. In many of the body's organs benign tumours are much commoner than cancers. So if CT scans were used for screening many people would have unnecessary further tests and possibly even operations (not to mention a lot of anxiety) because the scan has detected an abnormality that could have been cancer but was actually a quite harmless tumour that could safely have been left alone
  • a normal CT scan gives about 200 times more radiation to people than a chest x-ray and this does increase the risk of cancer development, albeit slightly.

As you say, there are now CT scans that can be done using much lower radiation doses but these have been used specifically to look for early lung cancer. Trials have been done using these scans but to reduce the risk of examining people unnecessarily only who were thought to have quite a high risk of developing lung cancer were included in these studies. People were thought to be at risk if they met the following conditions:

  • they were 55 or over
  • the had smoked at least one pack of 20 cigarettes a day for 10 years
  • they were otherwise well with no signs of cancer

The results from these studies suggest that CT scans used in this way do detect a number of early lung cancers which have not yet caused any symptoms. It is too soon to say whether or not this earlier diagnosis actually leads to an increase in the number of cures.

Certainly there is no evidence that scanning other parts of the body in people who are well and symptom-free, has any benefit in the early diagnosis cancer, leading to improved cure rates.

So, since you are currently well and do not have specific complaints a screening CT scan would not be recommended.

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.

Can you tell me the difference between a CT scan and an MRI scan? Is one better than the other?

CT scans became widely available in the UK about 20 years ago. The initials CT stand for computerised tomography. CT-scanners produce a specialised type of x-ray which builds up a three-dimensional picture of the inside of the body. The scan is painless. It takes longer than an ordinary x-ray (about 30 minutes) and involves lying on a couch which moves through a short tunnel, which houses the scanner. Most people who have a CT-scan are given a drink or injection of dye to allow particular areas to be seen more clearly. For a few minutes this may make you feel hot all over.

MRI scans became widely available in Britain during the 1990s. The initials stand for magnetic resonance imaging. The test is similar to a CT-scan but uses magnetic fields rather than x-rays to build up a series of cross-sectional pictures of the body. During the test, which may take longer than a CT-scan, you will be asked to lie very still on a couch inside a metal cylinder. It can be slightly uncomfortable and some people find it claustrophobic. The scan is also very noisy but you may be given earplugs or headphones to wear. Once again drinks or an injection of dye may be used to show up particular parts of your body.

Both types of scanner use computers to produce the pictures of your body and it usually takes a day or two to get the results of the scans.

At the present time there are many more CT-scanners in the country than MRI scanners but the numbers of both types of scanner are increasing all the time.

CT and MRI scanners are used to help diagnose and assess many different medical conditions. In cancer they are of great help in showing the exact position of tumours and helping to see whether or not they have spread.

In diagnosing and assessing cancers the two sorts of scanner are complimentary: for some cancers CT-scans will give more accurate information whilst for others an MRI scan is better. So the choice of which sort of scan is used depends on which type of cancer is suspected, or being investigated, and your medical team will be able to decide which of the tests is needed. Sometimes both CT and MRI scans are used to get even more information.

What does a CT scan show up?

A CT scanner is a very specialised type of X-ray machine.

The CT scanner passes X-rays through the body, from one side to the other. The different tissues will absorb different amounts of X-rays before they reach the detectors at the opposite side of the body. For example, air, which makes up most of the lungs, absorbs very little of the x-rays so a lot reaches the detector [which is why the lungs appear black on an X-ray film] but bone absorbs a lot of the x-rays, so little radiation reaches the detectors [which is why it appears white on an X-ray film].

The different amount of x-rays, or radiation, reaching each detector is processed by a computer and used to make detailed pictures of the body. These look very much as though the patient has been sliced into a series of horizontal sections, each a few centimetres thick. All the organs of the body can be seen in great detail. The X-ray specialists (radiologists) can tell if the organs look normal or if they look diseased. The organs may be too big, too small or the wrong shape. Often the way an organ looks can give a lot of information on what might be causing the problem.

The CT scanner can also pick up tissue that shouldn't be there, such as a tumour or swollen lymph nodes.

The radiologists sometimes use special dyes, called contrast, either into the vein or by mouth, to make it easier to spot any abnormalities. The contrast often makes the differences between normal and abnormal tissue clearer.

A great advantage of CT scans is that you can compare the pictures before and after treatment to see if there has been any improvement. You can also keep a check with scans to detect recurrences of tumour in places that are difficult to feel such as inside the belly or in the middle of the chest.

It is important to remember, however, that although they are very helpful, CT scans, like all medical tests, are not 100% reliable and occasionally will fail to spot an abnormality or may make something look abnormal when in fact there is nothing wrong.

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