In a new study published online Monday in the Journal of Clinical Oncology, researchers report that the rate of double mastectomy in women with single-breast cancer rose 150 percent in six years - even though the aggressive treatment doesn’t necessarily improve survival rates.

The researchers used a national cancer-treatment database to study the treatment decisions of more than 150,000 women who were diagnosed with cancer in a single breast from 1998 to 2003. White women, younger women and women whose cancer originated in their milk glands, increasing risk of occurrence in the other breast, were more likely to choose a double mastectomy, the researchers found.

“It’s a very vulnerable time for these patients. In a very short period of time they have to decide how to treat their breast cancer. Under the stress of the situation they will have both breasts removed because it seems logical at the time,” said lead author Dr. Todd Tuttle, chief of surgical oncology at the University of Minnesota.

The actual number of women choosing double mastectomy remains relatively small: Of all women undergoing mastectomies of any kind in the study, 4.2 percent chose double mastectomy in 1998, rising to 11 percent in 2003. A total of 4,969 women in the study received a double mastectomy.

The trend is troubling, Tuttle said. Cancer is more likely to spread to other parts of the body than it is to develop in the second breast. Women often make the irreversible decision too quickly, when they are under duress, he said.

“I often tell patients to wait, saying, ‘Let’s treat the cancer we know about and deal with that,’ ” Tuttle said.

One in every eight women will develop invasive breast cancer at some time in their lives, according to the American Cancer Society. In 2004, the most recent year for which federal data is available, nearly 187,000 women in the United States were diagnosed with breast cancer and nearly 50,000 women died.

Typically discovered through mammography or direct examination, breast cancer is treated with medication, chemotherapy, radiation, and surgeries ranging from lumpectomy, in which a part of the breast is removed, to mastectomy, in which the entire breast is removed and in many cases reconstructed with implants or tissue from other parts of the body.

Doctors and patients’ advocates urged women considering the procedure to go for counseling and take their time before making a decision, noting that other risk-reducing options were available.

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.