THURSDAY, May 8 (HealthDay News) -- While the rate at which breast cancer tumors grow varies among patients, that growth tends to be faster among younger women, Norwegian researchers report.

These findings may help in planning and evaluating screening programs, clinical trials and other studies, the researchers say.

Using a new mathematical model, the scientists were also able to estimate the numbers of breast cancers detectable by mammography. This is a new approach to estimating the growth rate of tumors and the ability of mammograms to find them.

"There are enormous implications for the sensitivity of breast cancer screening programs," lead researcher Harald Weedon-Fekjr, of the Department of Etiological Research, Cancer Registry of Norway, said in a statement.

"We found that mammography screen test sensitivity increases sharply with increased tumor size, as one might expect. Detection rates are just 26 percent for a 5 millimeter tumor but increase to 91 percent once a tumor is 10 millimeter in size," he added.

The report was published in the May 8 issue of the online journal Breast Cancer Research.

In the study, Weedon-Fekjr, and colleagues tested their model using mammography results from 395,188 women aged 50 to 69.

The researchers found that the growth rate of tumors varied significantly between patients. About one in 20 tumors doubled in size, from 10 to 20 millimeters in just over a month. However, a similar number of tumors took more than six years to double in size.

Based on this finding, Weedon-Fekjr's team estimated that it takes an average of 1.7 years for tumors to double in size. Moreover, tumor growth appeared to be faster among younger women and slowed as women aged, the researchers noted.

"Tumor growth and test sensitivity estimates can be directly linked to tumor size in a full population study, resulting in very useful growth estimates directly connected to a biologically relevant measure," the researchers wrote.

"Tumor growth seems to vary greatly between tumors, with higher growth rates among younger women. Most tumors become visible at screening when they reach a diameter of 5 millimeters to 10 millimeters," they concluded.

One expert thinks this study again confirms the need for women to have a mammogram every year.

"This study continues to prove why we need to screen women every year, starting at age 40," said Debbie Saslow, director of breast and gynecologic cancer at the American Cancer Society.

This is another study that shows that tumors grow faster in younger women, Saslow said. "It just doesn't make sense to have guidelines that say younger women should be screened every one to two years and every year as they get older," she said.

More information

For more on breast cancer, visit the U.S. National Cancer Institute.