Benefits of Breast Cancer Screening 'Modest at Best'

Roxanne Nelson

June 17, 2014

A new study adds more evidence that breast cancer screening can help reduce disease-related mortality rates.

Researchers from Norway found that screening reduced deaths from breast cancer by about 28%, which extrapolates to about 27 deaths avoided for every 10,000 women invited to biennial screening over a lifetime. The study was published online June 17 in BMJ.

However, this study largely confirms what is already known — that the benefits of routine breast cancer screening "are modest at best," note Joann G. Elmore, MD, from the University of Washington in Seattle, and Russell P. Harris, MD, from the University of North Carolina, Chapel Hill, in an accompanying editorial.

"While the benefits are small, the harms of screening are real and include overdiagnosis, psychological stress, and exorbitant healthcare costs," the editorialists write. Women need to be given balanced information, they note.

Data from older randomized controlled trials have not been ideal for determining the benefits and harms of "modern day screening," because much has changed since women first began enrolling in these studies, one of which dates back 50 years, Drs. Elmore and Harris explain.

Factors that influence breast cancer, such as hormone-replacement therapy and obesity, have changed. And increased awareness about breast cancer and improved technology have influenced patterns of diagnosis, they point out. In addition, treatment for breast cancer has improved considerably, which "may partially explain some of the benefit attributed to mammography."

Even though some have called for new randomized trials, the editorialists point out that these would take decades, and "further changes to risk factors, treatment, and technology over these decades" would likely make the new results obsolete once again.

Mixed Results

Multiple studies have attempted to evaluate the mortality benefit and associated harms of breast cancer screening with mammography. Although the results have not been consistent, several studies have shown a modest benefit from screening, such as the review that found that the breast screening program in the United Kingdom extends lives and that, overall, the benefits outweigh the harms.

In addition, a Dutch study found that even in the era of improved adjuvant therapy, population-based screening programs significantly reduced breast-cancer-related mortality.

However, a European study found the opposite — that although screening programs have been successful in reducing deaths from cervical cancer and colorectal cancer, the same does not apply to breast cancer. Rather, those researchers claim that the reduced death rates are the result of better treatment and healthcare delivery systems.

Reduced Mortality

In the Norwegian study, the researchers, led by Harald Weedon-Fekjær, PhD, from the Norwegian University of Science and Technology in Trondheim, compared the effect of "modern mammography screening" on screened and unscreened women.

All Norwegian women 50 to 79 years of age were evaluated from 1986 to 2009. A national screening program was initiated in 1995, and national coverage was achieved in 2005.

The researchers compared breast cancer mortality rates in women who were invited to screening with rates in those who were not. They made a clear distinction between diagnoses made before the first invitation for screening (without the potential for a screening effect) and those made after (with the potential for a screening effect).

A simulation model was used to estimate how many women 50 to 69 years of age would need to be invited to biennial screening to prevent 1 breast cancer death during their lifetime.

After 15,193,034 person-years of observation, fewer women who were invited to screening than who were not died of breast cancer (1175 vs 8996).

After adjustment for confounders, such as age, birth cohort, county of residence, and national trends in breast cancer death, the mortality ratio associated with being invited to screening was 0.72. To prevent 1 breast cancer death, 368 women would need to be invited to screening.

The effect of screening persisted after invitations ended (at 70 years of age), but declined gradually as time since last screening increased.

Could Be a Long Wait

This study "adds important information to a growing body of observational evidence estimating the benefits and harms of screening," the editorialists write. But it should also "make us reflect on how to monitor the changing benefits and harms of breast cancer screening."

Women need to be given balanced information to help them make informed decisions about screening, and physicians are more likely to mention the benefits of mammography screening than the harms, they note. "Future studies will hopefully allow analyses to account for changes over time in risk factors, screening technology, and treatment," Drs. Elmore and Harris explain.

But for "future independent boards to be able to conclude that the breast cancer screening decision has finally become obvious, careful assessment of ongoing screening programs will be required," they conclude. "In the meantime, make yourself comfortable — this may take a while."

The study was supported by the Norwegian Research Council. The authors have disclosed no relevant financial relationships. Dr. Elmore serves as a medical editor for the nonprofit Informed Medical Decisions Foundation. Dr. Harris is a former member of the US Preventive Services Task Force, which makes national recommendations on preventive care.

BMJ. 2014;348:g3701, g3824. Abstract, Editorial

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