Most men, diagnosed with prostate cancer, can delay or avoid harsh treatments and it will not harm their chances of survival, according to a new long-term study.

The study, published in The New England Journal of Medicine, showed that active monitoring of prostate cancer -- which involves regular tests to check on cancer -- had the same high survival rates after 15 years as radiotherapy or surgery.

On the other hand, the negative impacts of radiotherapy and surgery on urinary and sexual function persist and were found to be much longer than previously thought -- for up to 12 years. This suggests that active monitoring by healthcare professionals could be an equally valid -- and less harsh -- option.

"This is very good news. Most men with localised prostate cancer are likely to live for a long time, whether or not they receive invasive treatment and whether or not their disease has spread, so a quick decision for treatment is not necessary and could cause harm," says the lead investigator, Professor Freddie Hamdy from the University of Oxford.

"It's also now clear that a small group of men with the aggressive disease are unable to benefit from any of the current treatments, however early these are given. We need to both improve our ability to identify these cases and our ability to treat them," Hamdy added.

In the study, researchers from the Universities of Oxford and Bristol evaluated three major treatment options: active monitoring, surgery (radical prostatectomy) and radiotherapy with hormones for 1,643 men aged 50-69 years across the UK with localised prostate cancer.

They found that around 97 per cent of the men diagnosed with prostate cancer survived 15 years after diagnosis, irrespective of which treatment they received. Around a quarter of the men on active monitoring had still not had any invasive treatment for their cancer after 15 years.

Patients from all three groups reported similar overall quality of life, in terms of their general mental and physical health. But the negative effects of surgery or radiotherapy on urinary, bowel and sexual function were found to persist much longer than previously thought.

After 10 years of follow-up, men whose cancer was being actively monitored were twice as likely to see it progress or metastasise than those in the other groups. The assumption had been that this might lead to a lower survival rate for men on active monitoring over a longer time period.

However, the results from the 15-year follow-up show that this isn't the case and that survival rates remain similarly high across all groups, the researchers said.