Radiology: Mammo benefits dwarf radiation risks

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The radiation induced by clinically recommended mammography in women aged 40 to 74 results in an estimated 11 deaths per 100,000 women, while the screening saves the lives of 497 out of 100,000 women, a study to be published in the January issue of Radiology estimated.

Using data from various studies, the authors developed a model for predicting the risk of radiation-induced breast cancer resulting from mammography screening, arguing that the benefits of mammography screening far overweigh the risks posed by exposure to radiation.

"While the absorbed dose received by the breast during mammography represents a relatively small component of the lifetime accumulated dose from medical imaging and other sources, both the popular press and, frequently, the general medical literature tend to focus on the potential radiation risk from mammography, particularly as used for periodic screening," wrote Martin J. Yaffe, PhD, and James G. Mainprize, PhD, of the University of Toronto. "Although risk is mentioned frequently, this is usually done in nonspecific and qualitative terms."

Yaffe and Mainprize pulled data from the Canadian Cancer Society as well as various studies and statistics on life expectancy, breast cancer survival rates and average radiation doses to model the risks and benefits to lifespan of mammography screening. Lives lost and lives saved by mammography were calculated using algorithms and survival rates for 100,000 women aged 45 and over, with a 24 percent mortality rate applied for all breast cancer diagnoses.

The average radiation dose considered per mammography exam was 3.7 mGy. Women-years lost were calculated as the average number of years lost in a woman's life due to radiation-induced breast cancer multiplied by the disease's incidence within the 100,000-women cohort.

Mammography was estimated to save 497 lives and cause 10.6 deaths in 100,000 women aged 40 to 74 who were screened annually from 40 to 55 years and biennially from 56 to 74 years. Eighty-six mammography-induced cancers were predicted in the same cohort, resulting in an average number of woman-years lost of 0.0014, or 12.7 hours.

A total of 136 women-years were estimated to be lost due to radiation in the 100,000-person cohort, while 10,760 women-years were predicted to be saved.

The authors found that 87 lives per 100,000 would be saved by annual screening of women aged 40 to 49, while the radiation induced by screening lead to an estimated 7.6 deaths in this cohort. This finding equated to a benefit-to-risk ratio for lives saved of 11.4 to 1. The number of years saved versus cut short by mammography was 26.9 to 1 in women aged 40 to 49.

"The predicted risk of radiation-induced breast cancer from mammographic screening is low in terms of the number of cancers induced, the number of potential deaths and the number of woman-years of life lost," the authors summed.

Pointing out that their findings offered a more optimistic picture of mammography than other studies, Yaffe and Mainprize contended that this was the result of a lower radiation dose given the relatively recent switch to digital films, as well as updated (and improved) mortality figures for North American women diagnosed with breast cancer.

The authors affirmed their support for the clinical standard of annual or biennial mammography, concluding that "[f]or women 40 years of age and older, the expected benefit of reduction in premature mortality afforded by routine mammographic screening in terms of either lives saved or woman-years of life saved greatly exceeds this risk."