I did everything right. Why did I get cancer?

Risk is not the same as cause

A risk factor changes the probability of an outcome. It does not determine what will happen to one person.

Smoking is a clear example. It greatly increases the risk of lung cancer and several other cancers. However, not every person who smokes develops cancer, and lung cancer can occur in people who have never smoked. This does not weaken the evidence that tobacco causes cancer. It shows the difference between a strong risk factor across a population and the outcome for one individual.

The same principle applies in the other direction. Exercise, avoiding tobacco, limiting alcohol and maintaining a healthy weight can reduce the risk of certain cancers. They cannot reduce the risk to zero.

Why can people who have looked after their health develop cancer?

Cancer develops when a cell acquires biological changes that allow it to grow and divide when it should not. Usually, more than one change is involved.

Those changes can arise in different ways. Some are inherited. Some result from exposures such as tobacco smoke, ultraviolet radiation, certain workplace carcinogens or ionising radiation. Some infections increase the risk of particular cancers. Hormonal and reproductive factors are relevant to some cancer types. DNA copying errors can also occur as cells divide, even without an identifiable external exposure.

The balance between these factors differs greatly between cancers and between people. In many cases, there is no test that can separate their individual contributions after a cancer has developed.

This is sometimes described as chance or “bad luck”, but those phrases can be misleading. They may suggest that cancer is entirely random or that known causes do not matter. A more accurate description is that cancer biology includes processes we can identify, processes we can only estimate as risks, and events for which no specific cause can be found in one person.

Ageing and accumulated cellular changes

Cancer becomes more common with age. Cells have had more time to divide and to accumulate DNA changes. There has also been more time for environmental exposures to have an effect, and some of the body's protective and repair processes become less effective with age.

Age is therefore an important risk factor, but it is not a complete explanation. Cancer can occur in younger people, and different cancers have different age patterns.

Inherited susceptibility

Some people inherit a genetic variant that substantially increases their risk of one or more cancers. Inheriting such a variant does not make cancer inevitable. It changes the probability and may mean that fewer additional changes are needed before a cancer develops.

Most cancers are not explained by a single inherited variant. Cancer can appear more than once in a family because relatives share genes, environments or exposures, or because common cancers sometimes occur in several relatives without a hereditary syndrome.

If several close relatives have had related cancers, or cancers occurred at unusually young ages, discuss the family history with your clinical team. They can decide whether an assessment by a genetics service would be useful. Not everyone with cancer needs genetic testing.

Environmental and other exposures

Some cancer-causing exposures may have happened many years before diagnosis. They may have occurred at work, through air pollution, infection, radiation or other parts of the environment. The effect of a particular exposure may be difficult to measure, and some relevant exposures may never be identified.

Even when a known risk factor is present, that does not prove it was the sole cause of an individual's cancer. Cancer usually develops through several biological steps rather than one simple event.

Does prevention still matter?

Yes. The fact that cancer cannot always be prevented does not make prevention unimportant.

Avoiding tobacco, reducing alcohol, protecting skin from excessive ultraviolet exposure, maintaining a healthy weight, being physically active, receiving appropriate vaccinations and reducing harmful occupational exposures can lower the risk of particular cancers. Which measures matter most varies by cancer type and by country.

Prevention changes probability. Across a population, reducing common risks prevents cancers. For an individual, it improves the odds without providing a guarantee.

Could I have prevented this?

For most individual patients, it is not possible to say that one different action would definitely have prevented their cancer. Even a strong association between a risk factor and a cancer does not allow us to reconstruct exactly why that cancer developed in that person at that time.

There may be useful conversations about future health, treatment or risks for relatives. Those conversations should be based on the diagnosis, family history and evidence, not on assigning fault.

What matters now?

The practical priority is to understand the cancer you have and the options available. Useful questions include:

• What type and stage of cancer is this?
• Are any tumour biomarkers relevant to treatment?
• Is my family history clinically significant?
• What is the aim of treatment?
• What choices do I need to make now?
• Where can I get help with the emotional or practical effects of the diagnosis?

The biological features of a cancer also help explain why two people with apparently the same cancer may receive different treatment. Your team can explain which features matter in your case and what they mean for the treatment plan.