A microbeam which fires radiation particles at cancer cells one at a time is helping scientists to refine radiotherapy techniques.
Radiotherapy can have side effects
The Gray Cancer Institute says their work may lead to more potent treatment with fewer side effects.
The researchers have found the particles kill cells they hit directly - but also trigger them to tell neighbouring cells to commit suicide.
The work is published in the journal Cancer Research.
The researchers, funded by Cancer Research UK, were among the first to develop the microbeam, which fires a beam of helium ions just a thousandth of a millimetre wide.
In the new study, they grew brain cancer cells that they knew were highly resistant to conventional radiotherapy in culture dishes and targeted single cells with the beam.
Hitting just one cell among 1,200 sent a significant proportion of cells in the dish on the path to suicide.
Thus, by targeting just a few cancer cells, researchers were able to trigger large-scale waves of cell death.
Lead researcher Dr Kevin Prise said: "We used to assume that the only way to kill cancer cells with radiotherapy was to hit every one of the cells in the tumour with a fatal dose of radiation.
"Now we're finding that it's possible to hit just a handful of cells with much lower doses and let the cells' natural suicide machinery do the rest.
"Our discovery has important implications both for optimising the effectiveness of radiotherapy and for protecting healthy tissue from its effects."
The fact that a dying cancer cell sends out suicide signals to other cells has been dubbed the bystander effect.
Dr Prise said that if this effect could be enhanced within tumours it should be possible to develop more effective systems of radiotherapy - perhaps using lower doses to reduce side effects.
However, he said: "It also means that even very low doses of radiation may be doing more damage to normal cells than we'd thought, so we'll have to look for ways of protecting healthy tissue more effectively."
The researchers found the bystander effect relies on a molecule called nitric oxide, which plays an important role in a cell's response to stress.
When the amount of nitric oxide was reduced, cells targeted with radiation stopped sending out suicidal signals.
Dr Prise said: "Nitric oxide is a molecule cells produce to help them react to stressful situations and seems to be important in their decision to send out suicidal signals when they're hit by radiation.
"Making sure that there are high amounts of the molecule produced within tumours may be essential to optimise the bystander effect and improve treatments.
"We also think the mechanisms involved in the bystander effect might be different in healthy and cancerous tissue, so it might be possible to develop drugs that protect normal tissue from radiotherapy while leaving cancer cells more vulnerable."