Massachusetts General Hospital in the next few weeks will launch a large, long-awaited test of whether a controversial cutting-edge proton beam therapy is more effective than standard radiation treatment for prostate cancer.

Proton beam therapy, a targeted and controlled way to administer radiation to a tumor, has become a flashpoint in the debate over health care reform.

The expensive therapy is being used across the country and in some cases advertised directly to the general public before it has been deemed superior to standard radiation treatment, which costs about half as much. For years, doctors and federal health agencies have called for a scientific study like the one led by Mass. General, which will enroll its first patients by early June.

The five-year study will take place at a half-dozen centers across the country, including the University of Pennsylvania.

During that span, the number of therapy facilities, each of which can cost as much as $200 million to build, is projected to at least double across the country.

“Is the additional cost for proton beam therapy worth it?’’ said Dr. Jason Efstathiou, an assistant professor of radiation oncology at Harvard Medical School and Mass. General. “Ultimately, we need to figure out if these new emerging high-technology therapies being introduced into medical care provide a benefit… . This trial will determine whether or not the more expensive therapy leads to a better quality of life.’’

Proton therapy has theoretical advantages: It allows doctors to precisely target radiation to reach a certain depth in the body, which can reduce exposure and possibly mitigate side effects. But there is little consensus on whether that translates to better care, except in a handful of rare cancers - including pediatric cancers of the brain and nervous system or cancers in delicate areas such as the eye.