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Cambridge: Breast cancer treatment has come a long way, but one of the hardest questions has always been: who really needs chemotherapy, and who can safely do without it?

Chemotherapy saves lives, but it also comes with serious side-effects and long-term risks. A new study, presented at the American Society of Clinical Oncology’s annual meeting in Chicago, shows that a DNA-based test of the tumour can help two-thirds of certain breast cancer patients safely avoid chemotherapy without significantly compromising their long-term outcomes. Here’s what you need to know.

What is the new genetic test for breast cancer?

The new approach uses a genetic test to measure the activity of a set of cancer-related genes in the tumour.

Instead of just looking at the cancer under a microscope, the test looks inside the cancer cells and measures how strongly certain genes are switched on or off. From this, it calculates a score that reflects how aggressive that particular tumour is and how likely it is to come back after standard treatment.

Doctors then use this score to guide treatment decisions. If the score is low, it suggests the cancer is less aggressive and that surgery, radiotherapy and hormone tablets are enough, so chemotherapy can be safely avoided.

If the score is high, it signals a higher risk of the cancer returning, and chemotherapy is recommended because it is more likely to make a real difference. In other words, the test helps separate patients into those for whom chemo will genuinely help and those for whom it would be all harm and no extra benefit.

How significant is this?

This is a big step because it is based on a large, rigorous, late-stage clinical trial. For this particular, very common type of early breast cancer, the study shows that more than two-thirds of women who look high risk by traditional measures can safely skip chemotherapy if their gene test score is low, without losing protection against the cancer coming back.

That means fewer women being put through months of hair loss, nausea, fatigue, infection risk, early menopause, possible infertility and long-term heart or nerve effects when it doesn’t actually improve their outlook.

Who conducted this research and how large was the study?

The work comes from the Optima trial, led by researchers at University College London, involving multiple hospitals and cancer centres.

It focused on people with the most common form of early breast cancer – one that is fuelled by hormones rather than by a protein called HER2 – who would traditionally be considered for chemotherapy based on factors, such as the size of the tumour or whether it had spread to nearby lymph nodes.

More than 4,400 patients aged 40 and over were enrolled, making it one of the largest studies of its kind.

Participants were randomly assigned either to standard management, where chemotherapy decisions were based on the usual information doctors collect about cancer such as its size and appearance, or to a group where the Prosigna test result was used to guide whether chemotherapy was recommended. They were then followed for several years to see how often the cancer came back and how many remained cancer free.

When could this be available to patients?

The Prosigna test itself is not brand new. It is already approved and used in some centres. What has been missing until now is robust clinical trial evidence showing that using it to guide chemotherapy decisions is as safe as, or better than, current practice in a broad, real-world population.

In the UK, the next steps will be a detailed review by bodies such as the National Institute for Health and Care Excellence (Nice), the organisation that assesses which treatments the NHS should fund, followed by decisions about how widely the test will be offered.

That process takes time – usually months rather than weeks – but the expectation is that this will accelerate and broaden the use of Prosigna or similar tests for eligible patients, rather than leaving them as niche or optional extras.

The study shows a small survival gap between those who had chemotherapy and those who didn’t. Should patients be concerned about that?

The five-year survival rate was around one percentage point lower in the group that skipped chemotherapy – 93.7 per cent compared with 94.9 per cent.

Researchers considered this difference small enough to conclude that the gene-test approach worked just was well in practice, particularly given that the patients who avoided chemo were spared months of serious side-effects for no meaningful loss of protection.

For many people with a low-risk gene score, accepting that tiny difference to avoid chemo will be an obvious choice.

Others may feel differently. The key is that patients can now make that decision based on much better information about their own tumour.

What happens to the third of patients who still score high on the test and need chemotherapy?

Roughly one-third of the patients in the study had high Prosigna scores, indicating that their cancers were more likely to recur if treated with hormone therapy alone. For them, the recommendation remains to have chemotherapy as well as hormone treatment, because the potential benefit of chemo in reducing recurrence is significantly greater.

So this is not a story about getting rid of chemotherapy. It is about using it more wisely. The gene test helps protect those high-risk patients by flagging that they really do need chemotherapy, and at the same time it protects low-risk patients from being overtreated. In a sense, the test concentrates chemotherapy where it is most effective, which is better for both patients and health systems.

Does this apply to everyone with breast cancer?

No. The trial results apply to a specific, but very common, group: people aged 40 and over with early-stage breast cancer that is driven by hormones such as oestrogen, and which does not overproduce a growth protein called HER2.

Different subtypes of breast cancer behave very differently. For example, triple-negative breast cancer and HER2-positive breast cancer are often more aggressive and are treated with very different combinations of drugs, so this particular test and trial do not directly apply to them.

For younger patients under 40, and especially those in their 20s and early 30s, the biology and risk patterns can also differ. Genetic tests may still be useful, but treatment decisions are more complicated and must be tailored to each person.

So while this study is a big advance, it is not a one-size-fits-all solution, and it does not replace the need for careful discussion with a specialist team about each person’s overall risk and best treatment plan.