Doctors used shock waves to pulverize a woman's half-inch gallstone into thousands of particles last week, the first successful use in the United States of a machine they say could help thousands avoid gallbladder surgery.

Melissa Smith of Spartanburg, S.C., who underwent the procedure Wednesday at Baylor University Medical Center in Dallas, attended an afternoon news conference the next day and said she had no pain as a result of the 90-minute nonsurgical treatment. Gallstone removal normally requires surgery and up to five days of hospital recovery.

Doctors at Baylor and officials with Medstone Inc. of Costa Mesa, Calif., which produced the $1 million lithotripter machine used in the procedure, touted it as the first of its kind in the United States that did not also involve surgical or invasive procedures. The device has been used since 1980 to pulverize kidney stones.

"The sound wave therapy resulted in a fragmentation and breakup of the gallstone into thousands of pieces, some no larger than pieces of sand," said Dr. Ron Jones, chief of surgery at the Dallas hospital. "With the contractions of the gallbladder, the particles will be forced into the common bile duct and then into the intestines."

Jones said 15 million to 20 million people in the United States suffer from gallstones and 250,000 to 300,000 require surgery annually. He said the machine, which will be tested at seven hospitals nationwide, eventually could help up to half of those who would require surgery. "This method, in conjunction with drugs to soften the stone, may become the treatment of choice for these patients," Jones added.

Medstone is one of two companies approved for shock wave tests on gallstones. Dornier Medical Systems of Atlanta plans to start a similar series of tests in late February at 10 sites.