CHICAGO, NOV. 28 -- A high-energy "Star Wars" laser designed to blow nuclear missiles out of the sky shows promise as a medical instrument for blasting cancerous tumors out of the human body, a government researcher says.

The advanced free-electron laser, about 100 to 1,000 times more powerful than its nearest rival among other lasers, has been shown in animal studies to vaporize diseased tissue with pinpoint accuracy, leaving the surrounding flesh unharmed.

Such high power and precision, two qualities that have made the free-electron laser central to President Reagan's Strategic Defense Initiative, popularly known as Star Wars, will one day make the laser a useful weapon for safely destroying cancerous growths in humans, said John Madey, a Stanford University physicist overseeing development of the laser for the Defense Department.

"We probably have some months or years to go before we start limited clinical trials, but I have every reason to believe this could become a standard piece of medical equipment," Madey said in a telephone interview from Palo Alto, Calif.

"Therapeutic radiologists and laser physicians might consider it a new way of removing diseased tissue, while military experts hope the laser might knife through the heat shield and armor of enemy missiles approaching the United States from outer space."

Madey is to address an international meeting of radiologists in Chicago today. More than 34,000 scientists and technicians are expected to attend the six-day conference.

Madey developed the free-electron laser in 1972 under a Defense Department grant. The device later became a basic component in the SDI program. Several magnitudes more powerful than other existing lasers, the free-electron laser produces a powerful beam .01 inch to .04 inch in diameter.

Theoretically, a laser as powerful as the free-electron laser could be programmed to destroy incoming nuclear missiles from a base in outer space. But military application of the laser has been paralleled by medical interest. And preliminary animal tests suggest that it could replace surgical removal of many cancerous tumors. In one experiment, the laser drilled a perfect hole about .01 inch in diameter through a mouse's liver without damaging surrounding tissue.

Madey said the laser might also be used in other surgery and is capable of cutting through a quarter-inch of bone in three seconds.

Dr. Leonard Cerullo, director of neurosurgery at Northwestern University Medical Center, has been testing the free-electron laser on nerve tissue and is "very, very optimistic" about the results.