Quelle est la précision des tests de dépistage du cancer de la prostate ?

Cancer de la prostate is one of the most common cancers in the UK, affecting one in eight men at some point in their lives. While survival rates are good overall (84% of men diagnosed with prostate cancer in England and Wales will survive for more than ten years), it's really important for men to understand their risk.

The disease mostly affects men over 50, with the most common age at diagnosis being 65-69. It is twice as common in men of black ethnicity, and two and a half times as common in men with a family history of prostate cancer. Usually asymptomatic at the earliest, localised stage, it may eventually lead to urinary symptoms such as difficulty emptying your bladder, or needing to pee more than usual. If it has spread beyond the prostate, you may notice blood in the urine or semen, hip or pelvis pain, or unexplained weight loss.

It goes without saying that, if you do notice any changes of this kind, it's essential to visit the GP. No one symptom is really a giveaway for prostate cancer - they can all be caused by other health problems, most commonly non-cancerous l'hypertrophie de la prostate - so it may be necessary to run some diagnostic tests. Typically, the GP would start by offering you a PSA (prostate specific antigen) test.

"The PSA test is a blood test that can help to diagnose prostate problems, such as prostate cancer," says a spokesperson for Cancer de la prostate UK. "PSA is a protein produced by cells in the prostate and it's normal for men to have a small amount in their blood. The amount naturally rises a little as men get older and their prostate gets bigger. But a raised PSA level can be a sign of a prostate problem."

The pros and cons of the PSA test

You'll be entitled to this test if you’re aged over 50 (or over 45, if you have other risk factors). However, the GP will ask you to weigh up its pros and cons. While the PSA test has saved lives - it can help pick up cancer even before you have symptoms - its accuracy is far from perfect.

One in seven men with a normal PSA level may actually have prostate cancer, and in 2% of men that cancer is aggressive. The reverse is true too: three in four men with a raised PSA level don't have cancer, but will be put through significant worry and sometimes invasive testing as a result.

"PSA is produced by both normal and cancerous prostate cells, and your level can rise with other diseases," explains Dr Andrew Harbottle, chief science officer at MDNA Life Sciences. "You can have a high reading with things like benign prostatic enlargement, inflammation of the prostate, or even physical exercise, especially cycling. The PSA reading is an indication that there's something up with the prostate but it's not necessarily prostate cancer specific."

The Prostate Cancer UK spokesperson adds that this can cause a lot of anxiety and even unnecessary procedures. For one thing, if your PSA level is raised you may need a biopsy, which can have side effects. For another thing, you might be diagnosed with a slow-growing and low-risk prostate cancer, and end up having treatment you don't need.

"Most men in this situation now have their cancer carefully monitored instead, and only have treatment if the cancer starts to grow," adds the spokesperson.

Currently, if the GP thinks you need further tests, you'll be referred to a urologist. They'll arrange for you to have an MRI scan, followed by a biopsy if anything unusual is flagged up. In some hospitals, the first port of call would be the biopsy.

This diagnostic process, with all its advantages and disadvantages, is the best we have for now. However, Prostate Cancer UK is investing in a number of research projects that are exploring other routes of diagnosis.

"Recently there have been a number of studies highlighting areas to explore further," says the spokesperson. "These include imaging using different MRI and ultrasound techniques, tests based on measuring various proteins and DNA changes through blood samples, and tests for measuring tumour cells circulating in the blood."

One exciting possibility is a blood test, developed by MDNA Life Sciences. Called the Mitomic Prostate Test, the test analyses mitochondrial DNA to determine the presence, or otherwise, of cancer. The company says the test could reduce the number of prostate biopsies by up to 30%.

"Our test won't be replacing biopsies any time soon - it's more about giving the doctor information about which patients should be going forward for that biopsy," explains Harbottle. "This test would be used on men who have slightly elevated, but not super-high, PSA scores - what we call the 'PSA grey zone'. Three out of four of those men won't have cancer, so do you want to send them for a biopsy or not? Our test would be used to triage those patients, to see whether they do need an MRI-guided biopsy."

In a clinical trial conducted with the University of Cambridge, the test demonstrated a more than 99% negative predictive value (meaning that if the test came out negative, you'd be highly unlikely to have a clinically significant disease). It also showed 92% sensitivity, meaning men with a positive result could be fast-tracked for medical intervention.

Currently, the test is only available privately, but the company's partner, Aspire Pharma, is working to make it available on the NHS.

"They're in discussions with three hospital trusts to do a parallel study with the assay, to get the evidence that it works as we say it works and that it has an economic benefit to the hospitals," explains Harbottle.

The Prostate Cancer UK spokesperson sounds a note of caution for now, pointing out that until these studies have been conducted, we don't have enough information about its benefits.

"This innovative testing method is an interesting new approach, but there simply hasn't been enough detailed research into its effectiveness for us to be able to recommend for its use at this stage," he says. "Until then, we would not recommend that anyone use this test as a substitute for current standard methods."

Of course, research in the field will be continuing apace.

"In order to save more lives from prostate cancer it is crucial that we create a diagnostic process that is robust enough to routinely diagnose men early and accurately," says the spokesperson.