It only helps to find a breast cancer if it is already there.

Many women find having a mammography uncomfortable or even painful, but this is normally just for a short time. However, some women may find that the pain or soreness lasts for a few days.

All x-rays involve a certain amount of radiation, but mammograms only give a very low amount. The amount of radiation given during a screening appointment is about the same as the dose a person can receive on a plane when flying from the UK to Australia and back. There is less than a 1 in 23,000 risk of a mammogram causing breast cancer.

Therefore, the radiation risk associated with having a mammogram, is considered to be far outweighed by the benefits of detecting a breast cancer at an early stage, when it is more likely to be curable.

The radiation dose given by breast screening x-rays is continually monitored to make sure that it is as low as possible while still giving a good-quality image.

If you are concerned about the risk of having a mammogram, talk to the staff at the screening unit. You can find detailed information about the radiation risk of breast screening at www.cancerscreening.nhs.uk/breastscreen/publications/mp-rrbs-01.html  You can also telephone the NHS Response Line on 08701 555 455.

Mammograms can sometimes show an abnormal area in the breast. This can be very worrying, but often further tests will show that the abnormality is not a cancer. This is known as a false-positive result.

About 1 in 20 women who have breast screening will have an abnormal result and be asked to go for further tests. Approximately 7 in 8 of these women will not have cancer. The tests may include more mammograms, ultrasound scans or biopsies of the breast tissue, which can cause pain or scarring.

One or two mammograms in every 100 have to be taken again because:

• The x-ray picture is blurred and cannot be read by the doctor.
• There is some kind of equipment failure, for example the machine that processes the x-ray films breaks down (this is very rare).
• The mammogram misses part of the breast and it needs to be retaken so that the whole breast can be seen.

Although mammography is the most effective and reliable way of detecting breast cancer early, it is not perfect. A breast cancer may not be detected by mammogram because:

• Some cancers are very difficult to see on the x-ray.
• Some cancers, even though they are there, cannot be seen on the x-ray at all.
• The person reading the x-ray may miss the cancer (this will happen occasionally, no matter how experienced the person reading the x-ray is). To reduce the risk of a cancer being missed all mammograms are looked at by two people.

For the reasons given above, women having three-yearly breast screening may be diagnosed with breast cancer in the time between their screening appointments, even if they had a normal result after their last mammogram. This is known as an interval cancer.

If an interval cancer is diagnosed, the mammography films taken at the time of diagnosis will be compared to the previous mammograms. Women diagnosed with an interval cancer will always have the results of this review discussed with them. Research shows that the outlook and cure rates for women with interval cancers are better than for women who have never attended screening.

It is possible that a mammogram could find a breast cancer that would never have caused a problem during the woman's lifetime. Unfortunately, it is not possible to tell the difference between cancers that do cause problems and those that will not.

Research suggests that for every 200 women who have breast screening, one will have treatment that may not be needed. About 1 in 8 of all breast cancers detected would not have caused problems. This means that most women who are diagnosed with breast cancer as a result of breast screening do need treatment, because their cancers would have gone on to develop and cause problems without treatment.

It can be difficult to decide whether to have breast screening. The doctors and nurses at the breast clinic can give you more information to help you make an informed decision.

As breast screening can't pick up every cancer, you should see your GP if you have any unexplained changes in your breasts including lumps, pain and discharge from the nipples. You should see your GP even if you are having regular breast screening and have recently had a normal mammogram. 

There are many reasons for changes in breast tissue. Most of them are harmless, but you should always get them checked as there is a small chance that the changes could be the first sign of cancer.