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An "intelligent" knife that can sniff out tumours to improve cancer surgery has been developed by scientists.

The Imperial College London team hope to overcome the dangerous and common problem of leaving bits of the tumour in a patient, which can then regrow.

Early results, in the journal Science Translational Medicine, showed the "iKnife" could accurately identify cancerous tissue on the spot.

It is now being tested in clinical trials to see if it saves lives.

To avoid leaving cancerous tissue behind, surgeons also remove surrounding tissue.

They can even send samples off for testing while the patient is still in theatre, but this takes time.

Yet one in five patients who have a breast lump removed still need a second operation to clear their tumour. For lung cancer the figure is about one in 10.

The team at Imperial College London modified a surgical knife that uses heat to cut through tissue.

It is already used in hospitals around the world, but the surgeons can now analyse the smoke given off when the hot blade burns through tissue.

The smoke is sucked into a hi-tech "nose" called a mass spectrometer. It detects the subtle differences between the smoke of cancerous and healthy tissue.

This information is available to the surgeon within seconds.

Tests on 91 patients showed that the knife could accurately tell what type of tissue it was cutting and if it was cancerous.

Dr Zoltan Takats, who invented the system at Imperial, said: "These results provide compelling evidence that the iKnife can be applied in a wide range of cancer surgery procedures.

"It provides a result almost instantly, allowing surgeons to carry out procedures with a level of accuracy that hasn't been possible before.

"We believe it has the potential to reduce tumour recurrence rates and enable more patients to survive."

Trials are now taking place at three hospitals in London - St Mary's, Hammersmith and Charing Cross.

Prof Jeremy Nicholson, head of the department of surgery and cancer at Imperial College London, said: "This is part of what we call precision medicine, we're trying to change the world by very aggressively translating scientific discovery in to the NHS."

Surgeon Dr Emma King, of Cancer Research UK, said: "The iKnife is an exciting development to guide cancer surgeons during operations.

"If its usefulness is supported in further clinical trials, it could potentially reduce the time spent in theatre for many patients."