Why medical screening still has value

By Alan Katz

But not everyone should be screened

A version of this commentary appeared in the Huffington Post, Hamilton Spectator and the Guelph Mercury

Over the last few weeks there has been much debate in the media about the recommendations to limit population screening for two cancers. First came the US recommendation to stop routine mammography screening for breast cancer in women aged 40 to 49. This week saw another US recommendation: PSA screening for prostate cancer should no longer be routine care for men at average risk.

In fact neither of these American pronouncements should have been surprising or controversial in Canada. Many Canadian experts, including the Canadian Task Force on Preventive Health Care, have been making these same recommendations for years. The evidence, in other words, has spoken.

So many are now asking the question: if doctors are no longer supposed to screen in certain populations for two of the most common cancers, is the age of medical screening over? In a word, no.

Rather than put an end to medical screening screen altogether, experts are simply catching up with the evidence on which patients should undergo screening and which ones would be better served without.

Prostate screening: weighing the risks and benefits

Screening is the process where apparently healthy people are tested for a condition like prostate cancer. The screening test does not make the diagnosis, but a positive test — in this example, an elevated PSA level — suggests that cancer may be present and that the patient should undergo further testing.

Further testing for prostate cancer usually takes the form of a biopsy, where tissue is removed from the prostate gland and examined to determine if it contains cancerous cells.  The issue with screening is that these follow up tests are done on people without any signs of disease — they are healthy, in other words — so it is very important to err on the side of doing no harm.

But there is always a risk of doing harm in medical procedures.

When weighing options, doctors need to consider information about how likely an individual is to have a particular disease (in cancer screening, this is often influenced by age and family history), but they also need to consider what the consequences of further investigation might be for the patient. Not to mention, the consequence of not finding disease when it is present.

When cancer is present but not dangerous

There is a commonly held belief that all cancers are fatal if not treated.

For prostate cancer, the belief is that if we find the cancer early through screening we can save the patient’s life. We know that some patients die from prostate cancer, so the assumption is that all patients with prostate cancer will die from it if not treated in time. If this were true, it would make good sense to screen all men for prostate cancer. But we know that it is not true.

We know that the majority of men with prostate cancer will not die from it.

So does screening for prostate cancer save the lives of some men? Maybe. However, when studies have looked at large groups of patients it was found that PSA screening did not save lives. In contrast, the harms are almost universal.

Life changes immediately for every man diagnosed with prostate cancer. Most will develop erectile dysfunction from treatment (surgery or radiation), many will have trouble with bladder control. And many of these men will not benefit from treatment at all.

The dilemma is this: at present, we can’t differentiate those men who must be treated for prostate cancer in order to live, from those who don’t need treatment in the first place.

So we treat them all and cause most of them harm.

The new recommendations from the US against PSA screening for all healthy men over the age of 40 are simply catching up with what we already know.  Until we have a better screening test — and plenty of research is looking for just that — the harms of screening for prostate cancer outweigh the benefits.

Men with a strong family history of prostate cancer, and African American men, however, may still benefit from screening with the PSA test. And, of course, screening for other cancers, such as colorectal cancer and cervical cancer, saves many lives.

But as the evidence for prostate cancer screening of healthy populations signals, screening is not beneficial to everyone, and not everyone should be screened.

Alan Katz is an expert advisor with EvidenceNetwork.ca and Professor in the Department of Family Medicine at the University of Manitoba.  He is also the Associate Director of the Manitoba Centre for Health Policy, where he serves as the Director of Research. 

May 2012

This Commentary is from Commentaries, More Care is Not Always Better.

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