SCREENING, in other words early detection before symptoms appear, can be an important way to reduce the risks associated with some cancers.

For the national breast and cervical screening programmes, one of the roles of public health is to get as many women as possible to attend, as treatment is available and early detection gives better health outcomes.

Another public health role is to keep the screening programmes continually under review, to ensure that they are as effective as they can be.

In the case of breast screening, recently the age of routine screening has been extended, from 65 to 70 years.

Technical advances in cervical screening to be introduced in the next year or so will be accompanied by a change in the screening interval, depending on age - three yearly for women aged 25-49 and five yearly for women aged 50-64.

Most people seem to be very enthusiastic about being screened for more conditions, using new screening methods, which can be very good news for public health.

One of the next big programmes to be rolled out nationally will be screening for bowel cancer, which recently got a publicity boost from Prince Charles, such was the concern that the topic is still taboo in polite, as well as not so polite circles.

However, public health also has a role in controlling the introduction of new screening methods which do not appear to beneficial, and which may in fact do more harm than good.

Screening for prostate cancer falls into this category - surprising perhaps, but I will try to explain.

Every year, approximately 20,000 men in the UK are diagnosed with prostate cancer, most of whom are aged over 65, and 9,500 die from the disease.

The majority of prostate cancers grow slowly, over many years. By the age of 80, about half of all men will have some cancer cells in their prostate; mostly without any symptoms.

Many of the men diagnosed with prostate cancer will have these cells picked up if they have prostate surgery for unconnected 'water works' problems.

In short, most men who have prostate cancer will die with it rather than from it.

Prostate cancer presents major challenges for early detection through screening. Most enquiries that we get about prostate screening question why the PSA (Prostate Specific Antigen) blood test is not used more widely.

There are a number of reasons.

Firstly the PSA test isn't very good at telling between what is normal and what is abnormal - in other words, someone with a high level may not have cancer, whilst someone with a low level may have the disease.

Secondly, a high PSA result indicates a need for further tests - usually a needle biopsy of the prostate gland, which itself is not without risks.

Thirdly, if a diagnosis of prostate cancer is confirmed, some of the treatments currently available have significant risks, which in individual cases, may well be greater than would be caused by the disease itself.

This is particularly relevant when we know that most men who develop prostate cancer, will most likely not suffer any ill effects from it.

For all these reasons, the UK National Screening Committee does not currently recommend screening men for prostate cancer.

However, this position will be kept under review. Considering the importance of the condition, it seems likely that future developments in identification or treatment of prostate cancer will swing the balance in favour of screening, when the benefits of early detection are greater than the potential harms.