WASHINGTON - US health authorities issued guidelines Monday questioned the benefit of annual screening for breast cancer in women aged 40-49 and recommended only biennial mammograms for women over 50.

In an update of its 2002 recommendations, the US Preventive Service Task Force (USPSTF) said "the decision to start regular, biennial screening mammography before the age of 50 years should be an individual one."

After examining data from 600,830 women who underwent routine mammographs between 2000 and 2005, the USPSTF concluded that there was "moderate evidence that the net benefit is small for women aged 40 to 49."

The study said screening for breast cancer in the 40-49 age group can often lead to misdiagnosis and unnecessary surgery, or can fail to detect the cancer altogether.

In an earlier survey, the USPSTF said that in 10 percent to 20 percent of women diagnosed with breast cancer the tumors were not detected by mammography.

In another 10-20 percent of women, the task force added, growths were misdiagnosed as malignant, or cancerous, when they were benign.

Breast cancer is the second-leading cause of death among women in the United States. In 2008, 182,460 women were diagnosed with invasive cancer, 67,770 with non-invasive tumours and 40,480 women died from the disease.

The chances of developing breast cancer are one out of 69 for women aged 40-50, one in 38 for women 50-60, and one in 27 for women 60-70.