Scatter radiation from mammography does not increase risk of cancer

The radiation dose to areas of the body near the breast during mammography is negligible, or very low, and does not result in an increased risk of cancer, according to a new study. The results suggest that the use of thyroid shields during mammography is unnecessary.

During mammography, some x-rays scatter away from the primary beam in the breast and spread outward in different directions. Although this scatter radiation is much weaker than the primary beam, there has been concern that women exposed to it during mammography could face an increased risk of cancer, especially in radiosensitive areas like the thyroid gland.

To better understand the potential impact of scatter radiation, the investigators set out to measure the dose received by the thyroid gland, salivary gland, sternum, uterus, and the lens of the eye during screening digital mammography. Each of the 207 women in the study group wore six optically stimulated luminescent dosimeters to measure the absorbed dose of ionizing radiation while they were undergoing two-view screening mammography.

The dosimeters were analyzed by a medical physicist immediately after the examination, who found that the doses to the various areas outside of the breast ranged from negligible to very low.

The average estimated organ dose to the salivary gland was 0.05 mGy. The average estimated organ dose to the thyroid gland was 0.05 mGy. These doses are only a fraction of the radiation people are exposed to from natural background sources, such as cosmic radiation and radionuclides in the ground. In fact, all areas except for the sternum received less than 2% of annual background radiation dose.

Measured dose to the bridge of the eye and umbilicus was negligible, indicating no increased risk to the patient of cataracts or interference with normal embryonic development in early pregnancy.

The findings are particularly important in light of a recent increase in the incidence of thyroid cancer, one of the most radiosensitive of all cancers. The number of thyroid cancer diagnoses in women nearly doubled from 2000 to 2008, leading some to suspect that mammography may be a contributing factor and women should wear lead thyroid shields during examinations, an idea that the research team and other mammography experts strongly discourage.

Based on the extremely low scatter radiation dose to the thyroid, which is equivalent to just a few minutes of background radiation, thyroid shields are unnecessary during mammography. In addition, the researchers warn that use of thyroid shields could result in an increased radiation dose to patients.

“Thyroid shields can impede good mammographic quality and, therefore, are not recommended during mammography. A thyroid shield gets in the way of the exam and can actually cause an increase in radiation dose by necessitating repeat exams,” said Alison L. Chetlen, DO, of Penn State Hershey Medical Center.

Chetlen also pointed out that the thyroid gland is far less radiosensitive after age 30 years. The American Cancer Society and other organizations recommend that women have mammography screening once every year, beginning at age 40 years.

This study was presented at the annual meeting of the Radiological Society of North America.
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