Once inside a tumor cell, the genetically engineered virus can replicate and spread to other cancer cells while it leaves normal cells alone. In the study, patients experienced few side effects.

The results mean that "we have new approach to cancer treatment that would be much more selective" than current treatments, said study researcher John Bell, a senior scientist at Ottawa Hospital Research Institute in Canada. Such a treatment could be particularly helpful for treating metastatic cancers, which are cancers that have spread throughout the body.

The study was small, including just 23 patients, and as an early-stage clinical trial, it set a goal of testing the safety of the treatment rather than seeing whether the viruses could kill cancer cells or help patients live longer.

The researchers plan to conduct a larger study to test whether their therapy is more effective than current cancer treatments. "We will need to do more trials to know if this virus can truly make a difference for patients," Bell said.

The study will be published tomorrow (Sept. 1) in the journal Nature.