Regular monitoring of prostate cancer as a treatment option offers the same chances of survival 10 years after diagnosis as surgery or radiotherapy, a major study into the disease has discovered.
The decade-long trial, which examined men with localised prostate cancer, found survival rates were extremely high, approximately 99%, irrespective of the treatment administered.
There was no spread of the disease in around 80% of men who were actively monitored during the UK-wide study.
But having surgery or radiotherapy reduced the risk of cancer spreading even further, dropping by more than half against those being monitored, to less than 10%.
Researchers said the findings had "major implications" for men diagnosed with localised cancer via the prostate-specific antigen (PSA) blood test, used for cancer screening.
There is currently no routine screening for prostate cancer in the UK, and the study's results will play a key part in the decision on whether to screen for cancer.
But the scientists said the trial results alone would not allow them to recommend whether or not to implement such a programme.
Professor Freddie Hamdy from the University of Oxford, who led the research, said: "What we have learnt from this study so far is that prostate cancer detected by PSA blood test grows very slowly, and very few men die of it when followed up over a period of 10 years - around 1% - irrespective of the treatment assigned.
"This is considerably lower than anticipated when we started the study."
Prostate cancer is the most common cancer in men, with over 47,000 diagnoses every year in the UK and around 10,800 deaths. Between 20,000 and 30,000 men are diagnosed annually with localised prostate cancer through the PSA test.
The ProtecT trial, funded by the National Institute for Health Research and carried out by researchers from the Universities of Oxford and Bristol, is the first to examine the effectiveness, cost-effectiveness and acceptability of three major treatment options for men with localised prostate cancer - active monitoring with PSA testing every three or six months, surgically removing the prostate gland, and radiotherapy.
Between 1999 and 2009, 1,643 men aged 50 to 69 were given one of the three treatments after they were diagnosed via the PSA test, from a sample of 82,429 men, with localised prostate cancer.
The results, published in the New England Journal Of Medicine, revealed exceptionally low mortality rates, around 1%, irrespective of treatment type.
Surgery or radiotherapy reduced cancer spreading by more than half compared with active monitoring, occurring in less than 10% of men rather than around 20% for the monitored group.
Both interventions caused unpleasant side-effects, particularly in the first year post-treatment, but there was no difference in treatment effectiveness between them.
The research team said doctors and the NHS should consider the study's findings when discussing PSA-testing with men and any treatment options following a diagnosis of localised prostate cancer.
Prof Hamdy said: "The conversation now between doctors and patients will be much better informed than in the past and will have less bias than before. It will be more balanced and informed."
The findings are now being used to investigate the effectiveness and cost-effectiveness of PSA testing for screening of prostate cancer.
Professor David Neal, co-investigator from the Universities of Oxford and Cambridge, said the trial was of "global importance" and had "major implications" for men with PSA-detected localised prostate cancer.
But he added: "As far as the NHS is concerned it doesn't say there should be a screening programme or that there shouldn't."
Anne Mackie, director of Public Health England screening, said the results would give "key information" to men and their doctors when managing a prognosis.
But she added: "The problem with screening at the moment is that the test isn't very good and we don't know who to treat. What this trial is really important in saying is if you do have a localised cancer, here are the things that might happen and here are the choices that you can make."
Prof Hamdy said further study is necessary to determine the "trade-off" patients need to make between cancer outcomes and quality of life.
Dr Matthew Hobbs, from Prostate Cancer UK, called the discovery over survival rates "hugely positive".
He said: "At the moment, many men decide against active surveillance because of the uncertainty about the impact of that choice and the anxiety it causes.
"It is extremely reassuring to hear that, when it is performed to a high standard, active surveillance gives men the same chance of survival at 10 years as men who choose surgery or radiotherapy.
"It is important that these results are explained to men with localised prostate cancer, so they can weigh up the positives and negatives of each option, including side effects and risk of cancer progression, and be confident that they will make the best choice for them."
Dr Mackie said the results of the study would "help guide men and their clinicians about treatment decisions when they have a small or localised prostate cancer".
She added: "It does not change the situation in relation to a national screening programme.
"The recommendation of the UK National Screening Committee remains that a population-based prostate cancer screening programme would do more harm than good.
"This is because the PSA test is not a good enough test and there needs to be a better understanding about aggressive forms of the cancer and those that are not harmful.
"Many men will develop some form of prostate cancer that would not have caused harm in their lifetime - but might have unnecessary treatments as a result of screening that can have major side effects."