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. The expensive therapy is being used across the country 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.