Using a mathematical model to estimate breast cancer tumor growth rates based on screening data, Norwegian researchers found tumor growth rates varied considerably among women aged 50 to 69 years of age, with the fastest rates seen in the younger women.

The research is based on screening data from 395,188 women taking part in the Norwegian Breast Cancer Screening Program. It provides additional evidence for current American Cancer Society screening guidelines, which recommend yearly mammograms for every woman age 40 and older.

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

According to the researchers' estimates, it took an average of 1.7 years for tumors to double in size, but there was a great deal of variation among individual women. At the extremes, 5% had tumors that doubled in size in little over a month, while in another 5% it took more than 6 years.

While it's true that a women's risk of getting breast cancer increases as she ages, this study shows that younger women need to be vigilant about screening, as they may be more likely to have faster-growing tumors. Among study participants aged 50 to 59 years old, it took an average of 1.4 years for tumors to double in size, compared to an average of 2.1 years among women aged 60 to 69 years old.

Based on those findings, "it just doesn't make sense to have guidelines that say younger women should be screened every 1 to 2 years and every year as they get older," according to Saslow.

In addition to identifying a better model for estimating tumor growth based on tumor measurements, the researchers also looked at the sensitivity of current screening programs. The researchers found that most tumors become visible on mammograms when they reach 5 to 10 millimeters (mm) across. However, they write, "detection rates are just 26% for a 5 mm tumor and increase to 91% once a tumor is 10 mm in size."

The researchers hope their model will help improve these numbers. For more information on this topic, see the American Cancer Society document, Breast Cancer: Early Detection.