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Detector Mismatch Raises Radiation Exposure

Major Finding: Screening mammogram patients with correctly

matched breast and detector sizes received an average mean glandular

dose per breast of 3.3 mGy vs. 4.9 mGy for mismatched patients with

large breasts (P value less than .05).

Data Source: Quality assurance study in 886 mammography patients.

Disclosures: Dr. Wells and her coauthors reported having no conflicts of interest.


 

From the Annual Meeting of the Radiological Society of North America

CHICAGO – A mismatch between breast size and detector size during mammography resulted in significantly higher doses of radiation for women with large breasts in a study of 886 patients.

On average, women with large breasts screened on a small detector received almost 5 milligray (mGy) of radiation, which exceeds the American College of Radiology guidelines of 3-4 mGy or less for a standard two-view mammogram. When a mismatch occurs, women with large breasts receive significantly higher doses of radiation than women with small breasts or their counterparts with large breasts correctly matched to a large detector, Dr. Cathy Wells said when presenting the award-winning study at the meeting.

“Women with large breasts should be imaged with a large detector to avoid an unnecessary increase in radiation dose,” she urged.

The quality assurance study involved 886 women who presented for screening or diagnostic mammography during a 6-week period in late 2009. The exams were performed with a phosphor charge-coupled device detector, which is available in pre-set sizes (large or small) due to manufacturing constraints, she said. Insufficient data for 22 patients left 426 screening and 438 diagnostic patients evaluable for analysis.

A sizeable number, or almost 20% of patients, were affected by a mismatch between breast and detector size, said Dr. Wells, who completed the study at Beth Israel Deaconess Medical Center and is now a breast imaging fellow at Massachusetts General Hospital, both in Boston.

The percentage of mismatches varied from 10% of screening patients with large breasts, defined as a “C” cup or larger, to 27% of screening patients with small breasts imaged with a large detector. A mismatch occurred in 22% of diagnostic mammography patients with large breasts and 17% of diagnostic patients with small breasts.

Despite the sizeable number of mismatches in the study, not all women will be faced with this problem when they arrive for their mammogram, Dr. Wells said in an interview. The phosphor charge-coupled device detector is one of four types of digital detectors currently available in the United States, and to her knowledge the only type that has such size constraints. In addition, not all imaging centers use this detector type.

Some centers, including her own, have both large- and small-size detectors available, although there can be a wait for the proper size, she noted. Women can choose to wait or be imaged with a different detector after a discussion with the technologist.

“The best option for women to ensure a correct match between breast size and detector size would be to talk with the technologist who performs the actual mammogram, [as] the scheduler or person at the check-in desk will likely not know the answer,” Dr. Wells said.

“Women could ask the technologist whether the detector comes in different sizes, since not all do, and if so, whether they are correctly matched.”

Screening mammogram patients with correctly matched breast and detector sizes received an average mean glandular dose per breast of 3.3 mGy, compared with 4.9 mGy for mismatched patients with large breasts.

This was due to significantly more views obtained in mismatched patients with large breasts, compared with both the large-breast patients imaged on a large detector and small-breast patients imaged on a small detector (mean 5.9 views vs. 4.6 views vs. 4.7 views), Dr. Wells said. Interestingly, small-breast patients mismatched to a large detector underwent a similar number of views at a mean of 4.6, but actually received slightly less radiation at mean dose of 2.9 mGy.

During diagnostic mammograms, the radiation dose was again significantly higher among mismatched patients with large breasts, compared with the correctly matched large- and small-breast groups (8.2 mGy vs. 6.7 mGy, but it did not appear to be related to the number of views obtained, she said, adding that other factors must be at work. Several variables contribute to radiation dose, but in this case, the most likely culprit is compression thickness, Dr. Wells said. “It may be more difficult to adequately compress a large breast with a small detector, resulting in a larger radiation dose,” she said. “We hope to analyze the data again, to answer this question.”

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