Prostate cancer in some men is being allowed to progress to a dangerous stage because tests are not accurate enough, researchers suggest.
A study found more than half of a group of men whose tumors were initially classified as slow-growing and confined turned out to have a more serious form of the disease.
The findings, published in the British Journal Of Cancer, suggest that men are being given false hope by tests that underestimate the aggressiveness of their disease.
Around 41,700 men in the UK are diagnosed with prostate cancer every year and 10,800 die from the disease.
Many high-profile sufferers, such as Sir Ian McKellen, have spoken out in recent years to increase awareness of the cancer.
The findings cast doubt on a widely used a strategy of "active surveillance". The strategy involves monitoring patients with slow-growing tumors, rather than treating them with more radical therapy such as surgery or radiotherapy.
The research found that some of the tests that are used to carry out the "surveillance" fail to accurately rate how dangerous a cancer could be.
Urological surgeon Dr. Greg Shaw, from the Cancer Research UK Cambridge Institute, said:
"Our results show that the severity of up to half of men's prostate cancers may be underestimated when relying on tests before they have surgery. Whilst active surveillance would seem to be a safe approach for some men, nearly a third will end up needing surgery or radiotherapy within five years."