Yet the patients still were having thousands of episodes of ventricular tachycardia, during which a heart may beat 200 times a minute as blood pressure plummets. Defibrillators in their chests shocked their hearts back to a normal rhythm when necessary, but the devices did not prevent these episodes.

In the three months before they tried the experimental treatment, the five patients together suffered more than 6,500 bouts of tachycardia. It took a month for their hearts to recover from treatment, but in the year afterward the patients collectively had just four episodes of tachycardia, and two did not have any at all.

What made the new method possible was the fusion of two techniques. The first, stereotactic radiation, is used at some medical centers to treat cancer patients. It employs a highly focused and intense beam of radiation to destroy tumors.

The other technique maps the heart to pinpoint the exact location of scar tissue. The patients wears a vest made of chains of electrocardiogram leads — 256 in all, compared with 10 for the usual electrocardiogram. The doctors overlay data from the vest with images from scans of the patient’s heart.

“That gives us a beautiful three-dimensional image of where the arrhythmia is coming from,” said Dr. Phillip S. Cuculich, a heart rhythm expert at Washington University in St. Louis and first author of the new study.

Dr. Clifford G. Robinson, a radiation oncologist at the university and senior author of the report, was wary at first when Dr. Cuculich proposed using focused radiation to burn off scar tissue in malfunctioning hearts.

Oncologists go to great lengths to avoid the heart when they treat cancer, because radiation can damage valves, arteries and other cardiac tissue in ways that may not show up until years later. The trickiest part: hitting a moving heart in a breathing patient with a beam that must be precisely targeted.