Thousands of cancer patients may be dying unnecessarily each year because their doctors are giving them inadequate doses of drugs, researchers said today.

Doctors apparently are reducing the intensity of their patients' chemotherapy because of adverse side effects and fears of malpractice lawsuits, according to Dr. William Hryniuk of McMaster University in Ontario, Canada.

Dr. Vincent T. DeVita Jr., head of the National Cancer Institute in Bethesda, estimated that as many as 9,000 patients may die each year because they receive chemotherapy doses too low to be effective.

About 200,000 patients receive chemotherapy annually in the United States, including about 50,000 who could be cured, he said at an American Cancer Society seminar. He said that small reductions in dose intensity significantly reduce the treatment's effectiveness.

Meanwhile, a cancer therapy that caused an avalanche of calls from the public when it was announced in December continues to look promising, Dr. Steven Rosenberg, the National Cancer Institute's chief of surgery, said.

Rosenberg reported in December that the therapy, used on patients whose cancer had spread and for whom other treatments had failed, shrank the volume of cancer by at least 50 percent in 11 of 25 patients.

The therapy has been used on 49 patients, 18 of whom showed such regression, he said. With so few patients involved, Rosenberg said, "that statistically is equivalent, so there's been no change" in the response rate.

Regression was seen in all eight cases of advanced kidney cancer, with one cancer and possibly another completely gone, he said. About half the cases of malignant melanoma, a deadly skin cancer, responded similarly to the treatment, and one-fourth to one-third of patients with colon or rectal cancer responded, he said.

Rosenberg stressed that the technique still is experimental.