Tiny bubbles of fat in urine can help determine how aggressive a prostate cancer is, according to a study sponsored by Cancer Research, UK.
The study focused on fatty capsules called exosomes that are flushed out of the body in the urine.
Scientists found that in patients with prostate cancer exosomes contain molecules that come directly from the tumour itself.
These molecules, which contain a type of genetic material called RNA, can be used to figure out which genes are turned on and off in the cancer - and thus whether it is aggressive or not.
The researchers, led by Dr Jonas Nilsson, from the VU University Medical Centre in Amsterdam, hope the discovery will enable them eventually to develop a more effective test for aggressive tumours.
The findings appear in the British Journal of Cancer.
Dr Jonas Nilsson, lead author based at the VU University Medical Centre in Amsterdam, said: "We hope that this innovative approach to studying prostate cancer will reveal new biomarkers for aggressive tumors.
"Tumor-derived RNA is preserved in these capsules and gives us an insight into the genetics of an individual's tumor."
Dr Lesley Walker, director of cancer information at Cancer Research UK, said: "This technique is a fresh view on an old problem and could really help scientists find that elusive biomarker.
"It's still unclear what the best treatment approach is for early prostate cancer, so it's important we find answers to this as soon as possible.
"Distinguishing the aggressive tumors that must be treated from those that don't need treatment will go a long way towards resolving this issue."
Until now, researchers have used levels of proteins, like prostate specific antigen (PSA), produced by cancer cells to try to spot the aggressive tumours.
But this can throw up inaccurate results, and lead to people unnecessarily undergoing treatment which can have long-term side effects, such as incontinence and impotence.
Each year around 34,000 men are diagnosed with the disease, and around 10,000 die from it.
But while the disease can be a killer, in its more benign form it often requires nothing other than close monitoring and the patient often eventually dies from another, unrelated condition.
John Neate, of the Prostate Cancer Charity, said the study was a step towards finding a reliable way to identify aggressive forms of the disease.
But he warned it was a small study, and scientists would need to examine exosomes from a larger number of men before they could assess the reliability of the technique.
He also said the need to massage the prostate to increase the likelihood that the relevant molecules were released into the urine might reduce its acceptability as a mass screening tool.
Mr Neate added: "Nevertheless, this approach holds promise as a non-invasive test of malignancy that could help men and their doctors in the future."