The senior researcher behind the study, Professor Brian Butler from the Houston Methodist Hospital in Texas, said: 'We have created a vaccine with the patient's own cancer cells – a treatment that complements, and may even enhance, what we can achieve with traditional radiation and hormonal therapies.'
Professor Butler's team genetically modified the cancer cells to carry out this suicide programme by directly delivering the therapeutic genes into them using an adenovirus (a virus similar to the one that causes the common cold).
The delivered gene is derived from a herpes virus and encodes the protein thymidine kinase. When the modified cancer cells began to produce the thymidine kinase in the patients, they were given an anti-herpes drug that attacked the cancer cells and made them self-destruct.
Although the patient's immune system had been unaware of the tumour cells before this, once they began to be destroyed by the drug, it was alerted to their presence and launched an attack, clearing the cancerous cells.
A total of 62 patients with prostate cancer participated in the study, and they were divided into two groups depending on how far their cancer had progressed. Those in whom the cancer was confined to the prostate received two rounds of the experimental gene therapy. Patients with more advanced prostate cancer received three rounds of gene therapy along with hormonal therapy. All the study participants received radiotherapy as well as the experimental gene therapy.
Five years after treatment, 94 percent of patients in the first group and 91 percent of patients in the second group were free from cancerous cells. The researchers said that these numbers reflected a five to 20 percent improvement in outcome compared to previous tested therapies. The study was published in the Journal of Radiation Oncology.
Prostate cancer is the most common type of cancer in men with one in eight being affected over the course of their lifetime.
Professor Butler said that in future it may be possible to inject the virus straight into the tumour itself and 'let the body kill the cancer cells'.
'Once the immune system has knowledge of the bad tumour cells, if they pop up again, the body will know to kill them,' he told BBC News.
Professor Kevin Harrington from the Institute of Cancer Research, London called the findings 'very interesting', but pointed out that a weakness in the current study was that the viruses used to deliver the therapy cannot reproduce, unlike the next generation being investigated.
'It would be interesting to see this approach used with viruses that could reproduce to see if it makes for a more effective treatment,' Professor Harrington told BBC News.
A randomised controlled phase III trial of the experimental combination therapy is currently underway.