A new patient-friendly type of body scan could help doctors manage treatment for men with slow-growing prostate cancer.

A large number of men diagnosed with early forms of the disease currently go under "active surveillance" if their condition is not immediately life threatening.

Every year around 35,000 men are diagnosed with prostate cancer while an estimated 10,000 die annually from the disease.

During the "active surveillance" period, doctors will keep a close eye on a patient`s condition with biopsies and blood tests, with aggressive treatment only being introduced if the tumour starts to grow at a quicker rate.

But the standard PSA (prostate specific antigen) blood test can be unreliable, while biopsies - the removal of tissue samples for analysis - can be painful, invasive and have side effects.

Scientists believe that diffusion-weighted magnetic resonance imaging (MRI) would be a better alternative all round as it would be less invasive and provide more trustworthy results.

During the pilot study 50 patients were scanned after their initial diagnosis before being examined again two years later.

Each scan was used to calculate a figure called "apparent diffusion coefficient", a measurement of water movement within tissue.

Previously it had been shown that these measurements are significantly lower in men with high-risk prostate tumours.

By the time of their follow-up appointment, 17 of the men had received required treatment while 33 remained under active surveillance.

The team found the diffusion-weighted readings fell between the two scans in men who progressed to treatment, but remained constant in those who did not.

The findings have been published in the British Journal of Radiology.

Study leader Professor Nandita deSouza, from the Institute of Cancer Research, based in Sutton, Surrey, and London, said: "Diffusion-weighted MRI has a lot of potential for monitoring patients under active surveillance, as the scans clearly showed which men`s cancers were progressing.

"If the technique continues to show promise in larger-scale studies, it could one day save men under active surveillance from the discomfort and potential complications of regular biopsies."

Recent figures show the proportion of men opting for active surveillance rose from zero to 39% between 2002 and 2006.

It has become more common since the National Institute for health and Clinical Excellence (Nice), which looks at the cost-effectiveness of new drugs and procedures, allowed it to become a standard treatment option in 2008.

Lesley Walker, from the charity Cancer Research UK, said: "It`s important that we find better ways to distinguish prostate cancers that spread quickly and which could be fatal from those that may not even need treatment.

"Imaging like this has great potential to provide non-invasive, accurate ways to monitor patients to help doctors limit the number of men who undergo unnecessary treatment. It now needs to be confirmed in much larger studies before this test should be used routinely in a clinical setting."

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