Five per cent of breast cancer tumors appear to double in size in just over a month, Norwegian researchers said on Thursday in a study underscoring the potential benefits of more frequent screening.

The study published in the journal Breast Cancer Research also suggested detection rates of just 26 per cent for a five mm tumor, and 91 per cent for a ten mm tumor.

The researchers used a computer model fed with national screening and cancer data to calculate how quickly tumors grow and estimate the proportion of breast cancers detected at screening.

This data on nearly 400,000 women aged 50 to 69 helped them estimate that about five per cent of tumors may double in just over a month, growing from ten mm to 20 mm. This was mainly among younger women in the study's age group.

Another five per cent of tumors took more than six years to grow to the same size, according to the data collected before and after Norway began national breast cancer screening in 1995. This was mostly among older woman in the study.

"The variation was larger than what I was expecting," said Harald Weedon-Fekjaer, a statistician at the Cancer Registry of Norway, who led the study.

"Now we can be more certain about estimates."

The researchers used a complex formula that took into account what size tumors were when detected, measurements of surgically removed tumors and other data.

They said their findings could help in the debate over how often women should get mammograms. Some countries offer mammograms only once every three years and studies have provided conflicting evidence over whether mammograms save lives.

There is no question, experts say, that breast cancer detected earlier is far easier to treat, however.

Breast cancer is the leading cause of cancer deaths among women worldwide, according to the American Cancer Society. The group estimates about 465,000 women died from it globally in 2007, with 1.3 million new cases diagnosed.

Declining death rates from breast cancer in developed countries have been attributed to early detection through mammography screening and to improved treatment, it said.