Cancer screening is back in the spotlight following the results of a study exploring whether MRI scans could be used to pick up cases of prostate cancer more accurately and at an earlier stage than blood tests alone.

The short answer is that researchers concluded it could, though a much larger trial is now underway and the prospect of a national screening programme remains some way off.

At face value, screening makes perfect sense: why wouldn't you want to detect disease early, before it has the chance to cause ill health?

The same logic underpins the burgeoning popularity of so-called "health MOTs" offered by private providers.

The difficulty, of course, is the rate of false positives this throws up if applied universally to all adults regardless of age and risk factors.

This is exactly why, when it comes to the NHS, we do not screen everyone for everything.

READ MORE: Glasgow scientists to lead prostate cancer study which could stop disease in its tracks 

When it comes to cancer screening, the cost-benefit equation must balance the harms of overdiagnosis and overtreatment (picking up disease that either isn't there or is so low risk that it would never have developed into anything life-threatening, resulting in unnecessary exposures to radiation or surgical interventions which come with their own risks) against the potential gains.

How many people do you have to screen to detect a genuine positive case and, at a population level, how big an impact does screening make to overall mortality compared to never screening at all?

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Hopes for ovarian cancer screening were dashed in 2021 by the results of a 20-year trial which found that women aged 50 to 74 given regular blood tests for the CA125 protein - a potential marker for the disease - did not experience better survival rates than non-screened women.

Finally, you have to measure whether the survival benefit (the number of years a patient spends in good health following treatment) is merited by the cost of screening.

This is not always the case.

It is one of the reasons why we do not routinely screen women in their 80s and 90s for breast cancer, despite the fact that the incidence of the disease is 86% higher in the 85-89 age group than it is among 55-59-year-olds.

The goal of screening is to hit that "sweet spot" where the gains outweigh the harms and justify the costs.

Prostate cancer is now the most common cancer among men in Scotland and the rest of the UK, yet we currently have no routine screening programme.

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The impetus to develop one is such that the UK Screening Committee - the body which tells the NHS what to screen for, who to screen, and how - is currently evaluating six proposals, ranging from targeted screening for Black men aged 45 to 70 to screening all men aged 45 to 70 with frequency adjusted according to age, ethnicity, family history and their Prostate Specific Antigen (PSA) score.

The Herald: A much larger proportion of prostate cancer cases in Scotland are being detected at a later stage (blue and yellow lines) now compared to 2012A much larger proportion of prostate cancer cases in Scotland are being detected at a later stage (blue and yellow lines) now compared to 2012 (Image: PHS)

The research published this week was based on a sample of 303 men, aged 50 to 75, who were recruited via eight GP practices in London.

All participants underwent an MRI prostate scan and blood tests to measure their levels of PSA, a protein biomarker associated with prostate cancer.

In total, 64 screened positive for cancer although this reduced to 29 following NHS assessment.

Notably, 15 out of the 25 men identified in the study as having a positive MRI and clinically significant disease had returned PSA scores of less than three, meaning they would have been falsely given the all clear using the blood test alone.

The researchers noted that this "might explain why a single PSA-based screening confers so little impact on prostate cancer-specific and all-cause mortality".

Men over 50 can request a PSA test from their GP if they are worried about symptoms or have specific risk factors, but it is too unreliable for routine screening.

They added that using MRI scans instead could "pick up significant lesions before the PSA has begun to rise, and so offer an opportunity for early detection".

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However, they also note that it could lead to the "overdetection of cancers that will not become clinically relevant in a patient's lifetime".

Three of the cancer cases detected among the 303 volunteers were subsequently found to be "clinically insignificant".

The trial is now being scaled up to around 800 participants, with the potential for a national study in future.

READ MORE: Cancer waiting times now 'worst on record'

One of the obstacles, however, would be resourcing.

Even if MRI does detect prostate cancer early and accurately enough to be cost-effective, the risk is that it could backfire by overloading diagnostic pathways unless we have enough scanners to cope with demand and sufficient staff to deal with the extra imaging; at the moment we have neither.

This is particularly problematic given the UK's ageing population where the sheer number of people who will qualify for screening based on their age will inevitably increase.

The dilemma is summed up by bowel cancer screening, which targets 50 to 74s.

Between May 2020 and April 2022, a total of 29,228 kits returned for sampling in Scotland tested positive for possible bowel cancer, triggering an automatic referral for a colonoscopy.

The Herald: Thousands of patients are waiting over six months for colonoscopies, although suspected cancer patients are given priorityThousands of patients are waiting over six months for colonoscopies, although suspected cancer patients are given priority (Image: PHS)

Not everyone takes up that offer, but of the 21,769 who did roughly one in 20 (1,132 people) turned out to have cancer.

That is a lot of colonoscopies for a comparatively low hit rate.

The pay-off is that 61% were caught at an early, more treatable stage, but referrals will only increase and that has implications for waiting lists which have already ballooned by 47% since 2019.

By the end of March this year 5,679 people had been waiting over 13 weeks for a colonoscopy compared to 1,481 in March 2019.

READ MORE: Over-60s urged to return bowel cancer test kits 

The same quandary looms over the Screening Committee's recent recommendation to offer lung cancer screening on the NHS to all 55 to 74-year-olds with a history of smoking, using low-dose CT scans.

Rollout is already underway in England where pilot studies found that 76% of lung cancers detected via screening were caught in the earlier stages versus 29% for cases detected outside the programme.

This has genuine life-saving potential, yet Scotland, Wales, and Northern Ireland are no closer to implementation.

Why? Because we simply do not have the machines and manpower needed.