New research reveals why some prostate cancers are more aggressive, spread to different parts of the body and ultimately cause death.
It is hoped that the discovery, by the University of East Anglia in the UK, could transform patient treatment.
The findings come after the same team developed a test that distinguishes between aggressive and less harmful forms of prostate cancer, helping to avoid unnecessary, sometimes damaging treatment.
The new study shows how the number of "aggressive" cells in a tumour sample defines how quickly the disease will progress and spread.
The findings also reveal three new subtypes of prostate cancer that could be used to stratify patients for different treatments.
Lead researcher Professfor Colin Cooper, from the university's Norwich Medical School, said: "Prostate cancer is the most common cancer in men in the UK. It usually develops slowly and the majority of cancers will not require treatment in a man's lifetime. However, doctors struggle to predict which tumours will become aggressive, making it hard to decide on treatment for many men.
"This means that many thousands of men are treated unnecessarily, increasing the risk of damaging side effects, including impotence from surgery."
The team developed a test to distinguish aggressive prostate cancers from less threatening forms of the disease, by applying some complex maths known as Latent Process Decomposition.
In the latest study, published today, the team studied gene expression levels in 1785 tumour samples. They found that the amount of DESNT subtype cells in a sample is linked with the likelihood of disease progression - the more DESNT cells, the quicker the patient is likely to progress.
Co-lead researcher Dr Daniel Brewer said: "If you have a tumour that is majority DESNT, you are more likely to get metastatic disease. In other words, it is more likely to spread to other parts of your body. This is a much better indication of aggressive disease."