Doctors are looking for ways to boost radiotherapy

More than half of all cancer treatments involve radiotherapy.

The tumour is exposed to ionising radiation - and it works because it damages the DNA of cancer cells, triggering an auto-destruct mechanism.

However, research from a team in the US suggests that the real reason may be slightly more complex.

The study, published in the journal Science, found that damage to tiny blood vessels within the tumour also played a role in the shrinking of tumours.

Auto-destruct

The researchers, from Memorial Sloan-Kettering Cancer Center in New York, suggest that combining drugs that have a similar effect with radiotherapy might produce the optimum effect.

Doctors already know that radiation doses can produce damage to tiny blood vessels in surrounding healthy tissue - it is one of the side-effects of treatment that they try to minimise by focusing the radiation as precisely as possible on the tumour.

The US team had the reasonable theory that if this was happening outside the tumour, it was probably happening inside, as well.

Laboratory mice were genetically modified to lack a vital enzyme which has a key role in the "auto-destruct" sequence in type of cell that lines tiny blood vessels in the tumour.

If the change made it more difficult for radiotherapy to kill tumours, that would demonstrate that damage to blood vessels is a vital component of the overall regression of tumours under radiotherapy.

New approaches

Tests on the mice revealed that this was indeed what happening - the genetically altered mice did not respond as well to radiotherapy compared with normal ones.

Dr Zvi Fuks, one of the researchers, said: "Our results suggest posisble new clinical approaches.

"We need more research to look at several areas including radiation dose levels, timing of therapy, and possibly the combining of anti-angiogenic agents with radiation."

Dr Tony Elliott, who researches the effects of radiotherapy on DNA at Edinburgh University, told BBC News Online that it was likely that there was more than one effect at work to shrink a tumour after radiotherapy.

He said: "As yet we haven't managed to find anything that provides 'tumour kill' in the absence of the death of normal tissue.