A groundbreaking study has uncovered why some patients' cancers are more deadly than others, despite appearing identical.
Francis Crick Institute scientists developed a way of analysing a cancer's history to predict its future.
The study on kidney cancer patients showed some tumours were "born to be bad" while others never became aggressive and may not need treating.
Cancer Research UK says the study could help patients get the best care.
"We don't really have tools to differentiate between those that need treatment and those that can be observed," said researcher and cancer doctor Samra Turajlic.
One cancer could kill quickly while a patient with a seemingly identical cancer could live for decades after treatment.
It means uncertainty for both the patient and the doctor.
The work, published in three papers in the journal Cell, analysed kidney cancers in 100 patients.
The team at the Crick performed a sophisticated feat of genetics to work out the cancer's history.
It works like a paternity or ancestry test on steroids.
As cancers grow and evolve, they become more mutated and, eventually, different parts of the tumour start to mutate in different ways.
Researchers take dozens of samples from different parts of the same tumour and then work out how closely related they are.
It allows scientists to piece together the evolutionary history of the whole tumour.
"That also tells us where the tumour might be heading as well," said Dr Turajlic.
