19.6 — Screening and Prevention

Around 40 percent of cancers are preventable.

Which is a larger and more actionable figure than most people realise, and it is the most useful thing in this Part.

Screening adds to it — and screening is genuinely more complicated than "finding it early is always good", for reasons set out in Chapter 16.1.

What prevents cancer

In order of effect.

Not smoking. Responsible for around 20 to 25 percent of all cancer deaths, and not only lung: mouth, throat, oesophagus, stomach, pancreas, liver, kidney, bladder, cervix and some leukaemias.

And stopping works at any age. Stopping before 40 avoids around 90 percent of the excess risk; stopping at 60 still adds years of life expectancy.

Maintaining a healthy weight. Obesity is linked to at least 13 cancers (Chapter 18.8), and it is now the second largest preventable cause after tobacco in several countries.

Limiting alcohol. A Group 1 carcinogen — mouth, throat, oesophagus, liver, bowel and breast. And for breast cancer the risk begins at low intakes, which is not widely known. Less is better, and none is best from a cancer perspective alone.

Sun protection. Avoiding burning, particularly in childhood; no sunbeds (Chapter 14.5).

Vaccination. HPV prevents cervical, anal, oropharyngeal and other cancers (Chapter 15.13). Hepatitis B prevents liver cancer (Chapter 17.10).

These are the only two vaccines that prevent cancer, and they are among the most valuable interventions in this volume.

Physical activity. Associated with reduced risk of bowel, breast, endometrial and several other cancers, and with better outcomes after diagnosis.

Diet. A pattern high in fibre, fruit, vegetables and pulses, and low in processed meat. Fibre is protective for bowel cancer with reasonable evidence.

And the honest note: dietary supplements have not been shown to prevent cancer, and several have caused harm — beta-carotene increased lung cancer in smokers; high-dose vitamin E increased prostate cancer in one large trial (Chapter 16.2).

Treating infections. H. pylori eradication reduces gastric cancer (Chapter 9.2). Curing hepatitis C reduces liver cancer.

Avoiding occupational and environmental carcinogens — asbestos, benzene, radon.

Radon deserves a mention because it is invisible and common. A naturally occurring radioactive gas from the ground, accumulating in buildings, and the second leading cause of lung cancer after smoking in several countries. Testing kits are cheap and remediation is straightforward.

And breastfeeding, which reduces maternal breast and ovarian cancer risk (Chapter 15.8).

Screening: which programmes work

A screening programme is worthwhile only if it reduces mortality from the disease (Chapter 16.1), and surprisingly few do.

The ones with good evidence:

Cervical — the most effective cancer screening there is. Reduces cervical cancer incidence and mortality by around 70 to 80 percent.

And it works because it detects a pre-cancerous stage that can be removed, so it prevents cancer rather than only finding it (Chapter 15.3).

Now primarily HPV testing rather than cytology, which is more sensitive and allows longer intervals in those who test negative.

Colorectal — reduces mortality by 15 to 30 percent, and reduces incidence, by removing polyps (Chapter 19.5).

Breast — reduces mortality by around 20 percent, with the overdiagnosis trade-off discussed below.

Lung — low-dose CT in high-risk smokers reduces lung cancer mortality by around 20 percent. The newest programme, and being introduced.

Abdominal aortic aneurysm — not a cancer, and included because it is one of the clearest screening successes (Chapter 7.5).

And in specific groups: surveillance in Lynch syndrome, BRCA carriers, Barrett's oesophagus, and cirrhosis.

The ones that have not been shown to reduce mortality:

Ovarian cancer — the large UKCTOCS trial found no mortality reduction despite detecting cancers earlier.

Prostate — contested, and the balance has improved with MRI and active surveillance (Chapter 19.5).

Thyroid — actively harmful at population level, producing a fifteen-fold increase in diagnoses in South Korea with no change in mortality (Chapter 12.3).

Whole-body MRI and multi-cancer blood tests sold direct to consumers — currently unproven. They detect findings; whether they save lives is not established, and they generate substantial incidental findings and anxiety.

Multi-cancer early detection blood tests are in large trials, and they may prove valuable. They are not yet evidence-based, and marketing has run ahead of the data.

Why screening is harder than it looks

Three concepts, because they explain why "we found it early" is not automatically good news.

Lead time bias. Diagnosing earlier increases the time from diagnosis to death without changing the date of death. Survival statistics improve; nobody lives longer.

Length time bias. Screening preferentially detects slow-growing disease, because fast-growing disease appears between rounds. So screen-detected cancers look more survivable than they are.

Overdiagnosis. Detecting cancer that would never have caused symptoms or death.

And this is real rather than theoretical. Estimates for breast screening range from around 10 to 20 percent of detected cancers. The person is treated — surgery, radiotherapy, possibly chemotherapy — for a cancer that would never have troubled them.

And there is no way to tell which ones, which is precisely the problem.

Which is why screening decisions are properly presented as informed choices with numbers, rather than as instructions.

A useful framing for breast screening, from the numbers used in UK information leaflets: for every 200 women screened over 20 years, around 1 death from breast cancer is prevented and around 3 women are treated for a cancer that would never have caused harm.

Reasonable people weigh that differently, and both answers are legitimate.

Genetic risk

Around 5 to 10 percent of cancers are strongly hereditary.

When to consider genetic testing:

Cancer diagnosed unusually young.Multiple relatives with the same or related cancers.Multiple primary cancers in one person.Rare cancers — male breast cancer, or ovarian cancer at any age. Specific patterns — breast and ovarian together, or bowel and endometrial together. And certain ancestries — Ashkenazi Jewish ancestry carries specific BRCA founder mutations (Chapter 3.3).

What a positive result means:

Increased risk, not certainty. A BRCA1 mutation gives a lifetime breast cancer risk of around 55 to 70 percent, not 100 (Chapter 2.6).

Options: enhanced surveillance — MRI from a younger age; risk-reducing medication — tamoxifen or aromatase inhibitors reduce breast cancer incidence in high-risk women by around 30 to 50 percent; and risk-reducing surgery — which reduces breast cancer risk by around 90 percent and ovarian cancer risk substantially.

And cascade testing of relatives is where much of the value lies — each identified carrier leads to relatives who can act decades in advance.

The counselling matters as much as the test. Implications for insurance, for family relationships, and for reproductive decisions are real, and testing is done with support rather than as a laboratory transaction.

What to do practically

A short list, ordered by effect.

Do not smoke, and help others stop.

Take up the screening you are offered — cervical, bowel, breast, and lung if eligible. Uptake is the limiting factor in every programme.

Get the HPV vaccine, and get your children vaccinated.

Keep alcohol low.

Stay active and keep weight in a reasonable range.

Protect your skin.

Know your family history, and mention it.

And report the symptoms in Chapter 19.5 rather than waiting.

The delay between noticing something and seeking help is, across cancers, one of the largest single contributors to poor outcome — and it is entirely within an individual's control.

Most people who go to a doctor with a worrying symptom do not have cancer. That is the point: finding out costs an appointment, and not finding out can cost far more.

What the next page fixes

Chapter 19.7 covers treatment — surgery, radiotherapy, chemotherapy, targeted drugs and immunotherapy — and the change in the last decade that has been the largest in oncology's history.