An experimental "Trojan-horse" cancer therapy has completely eliminated prostate cancer in experiments on mice, according to UK researchers.
The team hid cancer killing viruses inside the immune system in order to sneak them into a tumour.
Once inside, a study in the journal Cancer Research
showed, tens of thousands of viruses were released to kill the cancerous cells.
Experts labelled the study "exciting," but human tests are still needed.
Using viruses to destroy rapidly growing tumours is an emerging field in cancer therapy, however one of the challenges is getting the viruses deep inside the tumour where they can do the damage.
"The problem is penetration," Prof Claire Lewis from the University of Sheffield told the BBC.
She leads a team which uses white blood cells as 'Trojan horses' to deliver the viral punch.
After chemotherapy or radiotherapy is used to treat cancer, there is damage to the tissue. This causes a surge in white blood cells, which swamp the area to help repair the damage.
"We're surfing that wave to get as many white blood cells to deliver tumour-busting viruses into the heart of a tumour," said Prof Lewis.
Her team takes blood samples and extract macrophages, a part of the immune system which normally attacks foreign invaders. These are mixed with a virus which, just like HIV, avoids being attacked and instead becomes a passenger in the white blood cell.
In the study, the mice were injected with the white blood cells two days after a course of chemotherapy ended.
At this stage each white blood cell contained just a couple of viruses. However, once the macrophages enter the tumour the virus can replicate. After about 12 hours the white blood cells burst and eject up to 10,000 viruses each - which go on to infect, and kill, the cancerous cells.
At the end of the 40-day study, all the mice who were given the Trojan treatment were still alive and had no signs of tumours.