AN alarm signal from the immune system offers an "invaluable" early warning of returning cancer, research has shown.

Scientists hope the discovery will make it possible to catch recurring cancers before they take hold, increasing the survival chances of patients.

The same team found that rousing sleeping cancers could weaken them and lead to their destruction by the body's own defences.

Knowing when a cancer is about to recur would allow doctors to attack a tumour before it begins to spread. Breast cancer is especially prone to making an unwelcome comeback - nearly a quarter of women with the disease will experience it a second time within 10 years.

Lead scientist Professor Alan Melcher, from the University of Leeds, said: "The ability to predict when a patient's cancer will come back would be an invaluable tool in treating the disease, allowing doctors to treat the recurrence rapidly and effectively before it takes hold. But we now need to find a way of using this knowledge to develop a test for patients whose cancer could take several years to reappear."

The main cause of cancer recurrence is small groups of tumour cells that lie dormant, only to re-awaken once they have learned to evade the immune system.

In a study of mice, researchers looked for early signs of an immune response triggered by a dormant cancer waking up. They found that detecting the signal made it possible to predict accurately when a cancer was about to return.

The scientists also learned it was possible to force a cancer out of its slumber prematurely. The immune system's defences could track down the cancer cells and wipe them out.

The findings are in the journal Nature Medicine.