Death rates from cancer are dropping more quickly across the United States, offering what an expert called a “glimmer of hope” against a leading killer.

According to a new report, cancer death rates fell by 2.1 percent each year from 2002 through 2004, almost double the 1.1 percent annual decline recorded between 1993 and 2003.

Most of the top 15 cancers in both men and women experienced declines in death rates. Notably, men saw declines in death rates for lung, prostate and colorectal cancers, while women saw declines in colorectal and breast cancer. In addition, the increase in death rates from lung cancer among women slowed considerably.

“That’s a very encouraging finding. It’s the key indicator of progress in cancer,” said Dr. David Espey, a cancer epidemiologist from the U.S. Centers for Disease Control and Prevention in Atlanta who was assigned to the Indian Health Service Division of Epidemiology and Disease Control, in Albuquerque, N.M.

The acceleration in decline of cancer deaths is “a good news story,” added Dr. Corey J. Langer, director of thoracic and head and neck medical oncology at Fox Chase Cancer Center in Philadelphia. “It’s the first glimmer of hope in a long time,” he said.

The report, which appears annually, is a joint effort from the American Cancer Society, the U.S. Centers for Disease Control and Prevention, the U.S. National Cancer Institute, and the North American Association of Central Cancer Registries.

Obesity is the single strongest risk factor for colon cancer in women, U.S. researchers reported on Monday.

They found that women with precancerous polyps in the colon were more likely to be obese than women without these lesions. And obesity more strongly predicted who would have these growths than smoking or having a family history of colon cancer.

“Of all the risk factors like age, family history, smoking, the most potent risk factor was being obese,” Dr. Joseph Anderson of Stony Brook University in New York, who led the study, said in a telephone interview.

“One in five lesions may be attributable to obesity.”

Colorectal cancer is the third-leading cause of cancer death in the United States. It will affect 153,000 Americans in 2007, according to the American Cancer Society, and will kill 52,000.

Family history, smoking and diet are all linked with colorectal cancer but Anderson said experts are still struggling to identify the causes that underlie most cases.

Doctors can use colonoscopy, in which a tiny camera is threaded up into the colon, to not only detect precancerous polyps but to remove them, thus often preventing cancer.

Anderson and colleagues examined the records of 1,252 women who underwent colonoscopy, classifying patients by age, smoking history, family history of colorectal cancer, and body mass index or BMI. Obesity was defined as having a BMI of 30 or higher.

Then they looked to see who had the most polyps, and who was more likely to have them at all.

“BMI was a huge risk factor. And it’s a risk factor that’s increasing,” Anderson said.

BMI was not linked to the risk of colon cancer for men, Anderson and colleagues found.

“We need to counsel people on things like losing weight and staying thin,” said Anderson, who presented his findings to a meeting in Philadelphia of the American College of Gastroenterology.

“Given the increasing number of obese patients in the United States, identifying them as high risk may have important screening implications,” he added.

Why obesity might be linked with colon cancer is unclear, said Anderson.

“Probably the leading factors are going to be insulin and insulin-like growth factor,” he said. People who have more visceral fat, the fat around the internal organs that is associated with the worst effects of being overweight, also have higher levels of insulin and insulin-like growth factor.