Drilling Down on Breast Cancer Screening Statistics

Breast cancer screening statistics standardized and applied to single scenario

/ Author:  / Reviewed by: Joseph V. Madia, MD Beth Bolt, RPh

(RxWiki News) What age should breast cancer screening start? How often should it be performed? How many women have to be screened to prevent one breast cancer death? Breast cancer screening is controversial.

After standardizing data from four major reviews to a common scenario, researchers have concluded that the benefits are clear and consistent — screening for breast cancer has saved lives.

This study synthesized estimates of the number of women who have to be screened in order to prevent one breast cancer death.

"Establish a breast cancer screening schedule with your doctor."

Robert A. Smith, PhD, senior director of cancer screening at the American Cancer Society in Atlanta, led a team of researchers in this review.

The goal of any cancer screening is to save lives. Taking that a step further, researchers and statisticians define the benefit more precisely by calculating the number of women who have to be screened in order to save one life.

And this is where the rub comes in. Depending on the study you look at, in order to save one life, anywhere from 111 to 2,000 women have to be screened. This is a 20-fold difference.

"We wanted to understand why these estimates differ so much," Dr. Smith said in a statement. "What we found was that the estimates are all based on different situations, with different age groups being screened, different screening and follow-up periods, and differences in whether they refer to the number of women invited for screening or the number of women actually screened.”

To distill these figures, Dr. Smith and colleagues took the data from four large reviews and applied them to a scenario used in the UK Independent Breast Screening Review.

The UK study analyzed the effect of screening women for 20 years, from age 50-69, on breast cancer mortality from age 55-79. This review estimated that 180 women needed to be screened to prevent a single breast cancer death.

The researchers used this scenario to standardize data from the Nordic Cochrane review, the US Preventive Services Task Force (USPSTF) review and the European Screening Network (EUROSCREEN).

The result of that synthesis found that an estimated 64 to 257 women needed to be screened in order to save one life, a substantial decrease from the original estimates of 111 to 2,000.

In terms of individual studies, the standardization dropped the estimates of women who had to be screened to prevent one breast cancer death as follows:

  • Nordic Cochrane review estimate dropped from 2,000 to 257.
  • USPSTF estimate fell from 1,339 for women age 50-59 and 337 for women age 60-69 to 193 for women age 50-69.
  • EUROSCREEN estimate dropped from 111 to 64.

“When we standardized all the estimates to a common scenario—i.e., the same exposure to screening, and a similar target population, period of screening, and duration of follow-up—the magnitude of the difference between studies dropped from twentyfold to about fourfold," Dr. Smith said.

Breast cancer screening advocate Daniel B. Kopans, MD, professor of radiology at Harvard Medical School and senior radiologist of the Breast Imaging Division at Massachusetts General Hospital, explained some of the reasons behind the differences.

"The studies that have been based on the randomized, controlled trials have not agreed, in part, due to analysts not realizing that the published figures are for women 'invited' to be screened. Not all women agreed to participate. The actual number of women who participated is much lower than the number who were invited, so that groups like the US Preventive Services Task Force that used the data on 'number needed to invite' miscalculated the 'number needed to screen'. This paper clarifies the errors and shows that the numbers are much lower than have been published," Dr. Kopans told dailyRx News.

Results from this study were presented at the 2013 San Antonio Breast Cancer Symposium. The study was also published in the November issue of Breast Cancer Management.

The Centre for Cancer Prevention, Wolfson Institute of Preventive Medicine, Barts and The London School of Medicine and Dentistry, Queen Mary University of London, United Kingdom funded this work.

No conflicts of interest were disclosed.

Review Date: 
December 11, 2013
Last Updated:
December 12, 2013