A prostate cancer test, which predicts how aggressive a tumour is, could spare men unnecessary operations, researchers suggest.
Early data, presented at the National Cancer Research Institute conference, suggests a genetic test can tell apart aggressive and slow-growing tumours.
A big challenge in treating the cancer is knowing whether surgery to remove the gland is needed.
Cancer charities said a successful test would be a "game-changer".
Prostate cancer is the most common male cancer in the UK. There are more than 40,000 new cases diagnosed and 10,000 deaths each year.
The decision to remove the prostate is based on an examination of a tumour sample under the microscope.
However, the procedure has significant side-effects such as infertility, difficulty maintaining and keeping an erection and uncontrolled urinating.
One of the researchers, Prof Dan Berney, from Queen Mary University of London, told the BBC: "We need a better test as we are over-treating many men; most will die with, not of, prostate cancer.
"We need to discriminate between the aggressive forms and those that will grumble along and just need monitoring."
The commercial test, developed by Myriad Genetics but independently assessed by Queen Mary University of London, looks at the activity level of genes inside a sample of the tumour.
If 31 genes involved in controlling how cells divide are highly active, it indicates the cancer is aggressive.
Prof Berney said such information could "substantially change" decisions made by doctors and patients but the costs were "huge" and it was certainly not going to be offered on the NHS in the next few years.
"We need to validate it and we're not there yet, but it is the strongest test we've had so far," he added.
Dr Iain Frame, director of research at Prostate Cancer UK, said: "Developing an effective test to distinguish aggressive from non-aggressive prostate cancer could be a game-changer for those affected by the condition.
"We urgently need to reach a point where we can focus resources on saving more of the 10,000 men who lose their lives to this disease every year, whilst sparing the many others who needn't have concerns.
"The results of this study are certainly intriguing, and take us a step closer to the diagnostic process for prostate cancer that men deserve. We will watch with great interest developments in this area."
Dr Harpal Kumar, the chief executive of Cancer Research UK, said: "Being able to tell apart aggressive and slow-growing tumours would help us take a major step forward in prostate cancer treatment.
"Understanding more about the nature of a patient's tumour could spare thousands of men from unnecessary treatment and the resulting side-effects, whilst also meaning that those who do need treatment receive it rapidly."