Prostate cancer is the most common cancer in British men and, at the age of 46, there is a 50-50 chance that I already have it. And I am not the only one who could be unwittingly harbouring the disease — prostate cancer is found at post-mortem in one in four men in their thirties, half of those in their fifties and in three quarters of 80-year-olds. So what am I doing about it? Not a lot. While a staunch advocate of screening programmes for cancer of the cervix, breast and bowel, I have reservations about screening otherwise healthy men for prostate cancer — reservations shared by many of my colleagues.

There are no reliable symptoms of early prostate cancer, and no official screening programme in the UK, but the Department of Health has just sent new guidance to GPs on the pros and cons of using the prostate specific antigen (PSA) blood test to pick up the disease.

PSA is a chemical produced exclusively by the prostate gland to liquefy semen. Higher than normal levels in the blood suggest an enlarged prostate gland and, in theory, the test should help to pick up prostate cancer in men who may be otherwise unaware that they have a problem, facilitating early diagnosis and treatment. But in practice the benefits are less clear-cut.

Two thirds of men with a raised PSA do not have cancer and as many as one in six of those who do will have a normal PSA. Or, to put in another way, one in six men with prostate cancer will be falsely reassured by the test, while two out of three of those who test “positive” risk being scared witless while being put through unnecessary, and often unpleasant, tests and investigations before being given the all clear.

It is hard to believe that modern medicine can ever be over zealous when it comes to detecting cancer, but in the case of the prostate gland that is exactly what the latest research suggests we are in danger of doing. According to a study in the British Journal of Cancer around half of all prostate cancers diagnosed by PSA blood tests would never have come to light during the men’s lifetime had they not requested screening.

While aggressive cancer of the prostate can be life-threatening, the majority of cases are slow growing and pose little or no direct risk to life. Unfortunately, the PSA blood test can’t differentiate between the two, and a positive test can initiate a chain of events that leads to some men being unnecessarily worried by a diagnosis that may never have troubled them, and even worse, subjected to unnecessary treatment, the most radical of which (surgery and radiotherapy) can leave them impotent and incontinent.

And herein lies another problem with screening for the disease — there is still some controversy about what to do with the men found to have cancer. While surgeons may agree on the benefits of radical surgery or radiotherapy in the minority of men with aggressive disease caught at an early stage, they are not so sure what to do with everyone else.

There are many examples — including some of my own patients — where PSA testing has turned up an otherwise lethal tumour and saved a man’s life but, contrary to the publicity they receive, such cases are the exception rather than the rule. Overall, the evidence that PSA screening saves lives remains equivocal.

At one extreme, sceptics argue that it influences survival only because it detects the disease long before it would otherwise become obvious. While at the other end of the spectrum, supporters point to data published in the New England Journal of Medicine which suggests that PSA testing can cut the risk of dying from prostate cancer by as much as 20per cent (although the same study found that 48 men had to be treated to save one life, meaning 47 underwent treatment that didn’t influence their odds of survival).

There is a glimmer of light on the horizon in the form of an alternative to PSA testing that examines urine for traces of a genetic marker (PCA3) produced in excessive quantities in around 95 per cent of prostate cancers. It is more sensitive than PSA, but is currently available only privately — at around £500 including crucial expert analysis of the results — and its role is still being evaluated.

Until such time as we have a better screening tool, and a clearer idea of who to treat and how to treat them, I am not going to put myself forward for testing. But that’s my decision — you must make your own. For more information on the PSA and PCA3 tests, and the treatment of cancer of the prostate visit www.prostateuk.org E-mail questions to drmark@thetimes.co.uk or write to times2,1 Pennington Street, London E98 1TT