Scientists are optimistic that they may have found a new treatment to help people with incurable brain cancer.
Ten patients so far in the UK have received the therapy, which is a virus that causes mild flu-like symptoms.
The virus can cross the blood-brain barrier and appears to help "switch on" the body's defence systems to attack the tumour, early studies suggest.
Experts at University of Leeds and four other centres now plan to treat more patients with reovirus therapy.
Although not a cure, the scientists hope it could be a useful add-on to traditional treatments, like chemotherapy and radiotherapy and newer immunotherapy drugs to buy patients more weeks, months or perhaps even years of life.
It is too early to know what impact, if any, reovirus treatment has on survival, but researchers are hopeful that with more studies they will be able to find out.
Dr Colin Watts, Cancer Research UK's brain tumour expert, told the BBC it was "an exciting first step along the journey towards clinical use".
He added: "Scientists working with surgeons and oncologists have proven that the virus penetrates into the tumour and does what it is supposed to do – wake up the immune system to see the cancer.
"Now clinical trials are seeing if that wake-up call is sufficient to kill the cancer cells and help to improve survival of patients with brain tumours."
How it works
The virus can be injected into a person's bloodstream rather than directly into the brain, which doctors say should be less risky and more convenient for the patients who receive it.
Reovirus tends to infect cancer cells and largely leaves healthy cells alone, say researchers. Patients receiving the treatment reported only mild flu-like side-effects.
Until now, scientists thought it was unlikely that the virus would be able to pass from the blood into the brain because of the protective membrane that surrounds the brain – the blood-brain barrier.
The first-ever patient trial of the treatment – in nine volunteers with fast-growing gliomas that had regrown despite surgery and chemo and radiotherapy, or advanced cancers that had spread to the brain from other sites – showed reovirus crossed successfully to reach its target.
Analysis of tumour samples suggested that the virus helped alert and ramp up the body's immune system to attack the cancerous tissue.
Prof Alan Melcher from the Institute of Cancer Research, who is co-author of the study published in the journal Science Translational Medicine, said: "Our immune systems aren't very good at 'seeing' cancers, partly because cancer cells look like our body's own cells, and partly because cancers are good at telling immune cells to turn a blind eye. But the immune system is very good at seeing viruses.
"In our study, we were able to show that reovirus could infect cancer cells in the brain. And, importantly, brain tumours infected with reovirus became much more visible to the immune system."
The findings have prompted other doctors to recruit more patients to try out a full course of the treatment.
Susan Short, professor of clinical oncology at the University of Leeds, has already started treating one patient in her care.
The man, who was only recently diagnosed with a very aggressive type of brain tumour called glioblastoma, is receiving multiple doses of reovirus treatment alongside standard therapy to see what effect it will have.
Prof Short said: "Brain cancer is a devastating disease. For a long time, there have not been many new developments that we could offer patients.
"We do not yet know how much of a difference the treatment will make because this is a new method that has not been available before. It's a paradigm change."
She said there were early hints that other "relatively harmless" viruses might work even better to prime the body to fight harder against these very aggressive cancers.
"We hope it might ultimately lead to a therapy that has a big impact, but we just don't know yet."
Sarah Lindsell, chief executive of The Brain Tumour Charity which co-funded the research, said: "Brain tumours cost too many lives. The only way to change that is through research. This news from the University of Leeds and the Institute of Cancer Research will be welcomed by all of those who know only too well the devastation caused by this cruel disease."
According to figures from Cancer Research UK, almost 11,000 new cases of primary brain cancers are diagnosed in the UK each year.
Only 14% of patients survive for 10 years or more following a diagnosis of a primary or malignant brain tumour.