Joe Biden’s rather sudden diagnosis of aggressive prostate cancer – which has already spread to his bones – is something of a medical mystery.

Doctors announced the rather grim diagnosis (as it’s spread to the bones it’s no longer curable) yesterday, just days after the 82-year-old former president of the United States complained of ‘urinary symptoms’ – most likely difficulty peeing due to the tumour squeezing the urethra, the tube that carries urine out of the body.

Yet during his time in office and indeed since, it’s almost certain he would have had regular health checks that would have included blood tests every few months or so.

These blood samples would have been used, among other things, to test for levels of prostate specific antigen, or PSA, a protein which can increase in the blood when prostate cancer is present.

At the first sign of a spike in PSA, his doctors would likely have recommended a biopsy (where minute samples of prostate tissue are removed, using a long needle, to be examined under a microscope for signs of cancer) as well as an MRI scan to see if a tumour was visible on the prostate.

This is standard practice in the US, where the American Cancer Society recommends every man over 50 has an annual PSA test.

It means that there is a better chance of catching any cancer early, successfully treating it and saving lives.

So the question is – was his cancer missed, or covered up?

It was revealed yesterday that former US president Joe Biden has been diagnosed with aggressive prostate cancer 

The cancer has already spread to the 82-year-old’s bones, meaning it is no longer curable

I don’t buy the theory that White House officials kept it under wraps while he was president, in order to ward off even greater public concern about the health of a leader who was clearly ailing.

I’ve worked in the US and the culture there is much more open about health problems and especially the need to treat cancer aggressively, hitting it hard and early with tumour-shrinking treatments.

Yet there is no obvious medical explanation for this apparently very sudden onset of widespread cancer.

One possibility is that Mr Biden’s regularly monitored PSA scores failed to show enough variation to worry his medical team or raise suspicions that cancer might be present.

PSA levels don’t just go up when cancer is around. Ageing is also a factor, as are urinary infections.

A ‘healthy’ score in men under 50 is around 2.5 nanograms per millilitre (a measure of how much of the protein is found in a millilitre of blood). But in those over 70, doctors are unlikely to be concerned unless it tops 6.5ng/ml.

PSA testing is the best method we currently have for detecting possible cancer but it’s not perfect.

In fact, studies suggest up to one in seven men diagnosed with prostate cancer has a ‘normal’ PSA level. And this may explain why Joe Biden’s tumour seemed to slip through the net.

The other possibility is this is an ‘interval’ cancer – an aggressive and fast-growing tumour that pops up in between PSA testing or scans.

The term is usually used in the context of breast tumours that appear in women who have had no sign of cancer at their three-yearly NHS screening appointment but develop a tumour before the next check is due.

It may be Mr Biden’s last PSA reading was normal and his cancer was only detected after he developed symptoms such as back pain leading to an X-ray showing tumours in the bone.

For an aggressive cancer of this nature to appear out of nowhere in a matter of months – maybe even weeks – is not impossible (although cancers do tend to be slower growing in older men) but the odds are stacked against it in someone as closely monitored as the president of the USA.

Either way, his cancer is not curable.

Doctors have allocated it a Gleason score (a sliding scale of prostate cancer severity) of nine out of ten.

They did this by analysing numerous prostate tissue samples under a microscope. Each one would have been scored one to five for cancerous appearance, in other words how different they look to normal cells and their behaviour or aggression – then they add the two most commonly occurring grades together to give an overall score out of ten.

Mr Biden’s total of nine means his cancer has taken a firm hold and no drugs – not even the modern immunotherapy medicines that have transformed some areas of cancer treatment – can reverse it.

However, it seems his particular type of tumour will respond to drugs that suppress his levels of the male sex hormone testosterone, which is known to feed the growth of some types of prostate cancer. This will slow the tumour’s expansion in the prostate and beyond and give him some quality of life but the reality is his life expectancy is probably no more than about 18 months.

It’s a reminder that prostate cancer is primarily a disease of older men – the average age at which men in the UK are diagnosed is 72.

But it’s important to remember that, for most men who get it, this is not how it typically plays out.

Many prostate cancer patients have slow-growing tumours that cause them no ill health or unpleasant symptoms and don’t actually kill them – they die of something else before the tumour gets a chance.

We call these tumours ‘pussycats’.

Joe Biden’s, I’m sorry to say, appears to be more of a tiger.

• Karol Sikora is the former head of the World Health Organisation’s cancer programme and an NHS oncologist for 45 years.

Interview: Pat Hagan