Breast cancer survivors may soon know exactly how likely their cancer is to come back.
A new test offers women peace of mind through predicting their risk of getting breast cancer again in the next 20 years.
Scientists have examined tumours to find there are 11 different types of breast cancer, all with separate risks of relapsing.
They can now identify those women who do not need to worry because they are in a group mostly likely to be cured after five years.
Simply by looking at genes in breast cancer tumours, they can also break the news to some women that their cancer has a 62 per cent chance of coming back in 20 years.
The good news for these patients is that doctors can then keep a closer eye on them, or keep them on the drug tamoxifen which makes it less likely their breast cancer will return.
More than 14,000 breast cancer survivors in Britain are believed to fall into this unlucky 'late-relapsing' group.
The test, developed by a team including the University of Cambridge and Stanford University, was developed by analysing the tumours of nearly 2,000 breast cancer patients and tracking their health over an average of 14 years.
Researchers say a simpler version could be available for doctors to use within five years.
Professor Carlos Caldas, lead researcher at the Cancer Research UK Cambridge Institute, said: 'Treatments for breast cancer have improved dramatically in recent years.
'But unfortunately for some women, their breast cancer returns and spreads, becoming incurable.
'For some, this can be many years later - but it's been impossible to accurately predict who is at risk of recurrence and who is all clear.
'In this study, we've delved deeper into breast cancer molecular subtypes so we can more accurately identify who might be at risk of relapsing and uncover new ways of treating them.'
Currently doctors work out if someone's cancer will come back by looking at things like the tumour size and aggressiveness, the patient's age, their type of breast cancer and time spent in remission since surgery.
But scientists can be far more precise by looking at genes in their tumours.
Multiple copies of genes that trigger cancer, or help cancer cells to divide, put unlucky women with two types of breast cancer into an 'aggressive' group with a high chance their cancer will come back.
In women with the deadliest form of breast cancer, called 'triple-negative' breast cancer, scientists found some have a tiny risk of this cancer coming back if they survive the first five years after treatment.
But for women with the same cancer but more unlucky genes, the risk of getting breast cancer again is 50 per cent higher after 20 years.
The same lottery was found for a type of breast cancer affecting three-quarters of patients, where tumours are 'oestrogen receptor-positive', meaning they need the hormone oestrogen to grow, but negative for a protein called HER2.
There were eight groups of patients for this cancer type - four with a good chance that their cancer would not come back.
But the quarter of women with this type of cancer who fall into the other four groups have a risk of 47 to 62 per cent of getting breast cancer again in 20 years.
Experts believe that people relapsing after this type of cancer do so because they do not respond well to radiotherapy, chemotherapy and drugs designed to eradicate cancer cells from the body.
These can come 'back to life' many years later, causing someone to get cancer again.
The researchers also worked out relapse rates for women with HER2-positive breast cancer, but say these are out of date because the patients in the study did not have access to a relatively new drug called Herceptin.
The test gives women their chance of breast cancer coming back after 10, 15 or 20 years and their chances of dying from breast cancer or another cause.
The results, published in the journal Nature, also reveal triple-negative breast cancer is more likely to spread to the brain, while oestrogen receptor-positive, HER2-negative cancer more often spreads to the bone.
Next researchers will investigate different treatment options for the 11 breast cancer groups they have found.
Professor Karen Vousden, chief scientist for Cancer Research UK, said: 'One in seven women will get breast cancer in their lifetime in the UK, and we hope that research like this will mean that if faced with the disease, even more of our daughters and granddaughters will survive.'
Source: The Daily Mail