Scientists say they have found out more about how prostate cancer manages to spread away from the gland to the rest of the body.
Radiotherapy is one option for prostate cancers
Prostate cancer which is confined to the gland is unlikely to kill the patient.
It is only when cancer cells break out into nearby blood vessels and are carried elsewhere in the body that the disease becomes far more serious.
The prostate gland is found in men close to the bladder, and makes a component of semen.
It is one of the most commonly diagnosed cancers, and kills thousands of men every year in the UK.
Some prostate cancers never spread and cause problems for the patient - while some are highly aggressive and spread quickly to other sites around the body.
However, scientists still do not understand what differences hold the key to this.
Many cancer cells enter the blood stream and don't go on to form successful metastases.
Dr Evan Keller, University of Michigan
Scientists at the University of Michigan in the US believe they have found a gene which plays a crucial role in determining whether a cancer stays put or "metastasises" - spreads outside the original tumour.
The gene produces a protein called RKIP, which appears to hold back the spread of cells into nearby blood vessels.
When a tumour produces a normal amount of RKIP, they discovered, it cannot make this jump.
However, in tumours lacking RKIP or only making tiny amounts, metastasis takes place.
The finding could help doctors determine which prostate tumours are likely to spread fast and need aggressive treatment - and which are less likely to spread, and can be treated more conservatively.
Often, this could mean the difference between radical surgery to remove the prostate gland - an operation which could affect bladder control and sexual function - or a "wait and see" policy.
If RKIP does turn out to be the key to prostate cancer spread, it is possible that tumours could one day be treated to change their genetic makeup - and reduce the chance of spread.
However, Dr Evan Keller, who led the study, said that cells entering the bloodstream was not the only factor which decided whether the disease spread.
He said: "Many cancer cells enter the blood stream and don't go on to form successful metastases."