Applicability of ACR breast dosimetry methodology to a digital mammography system

Determination of mean glandular dose (MGD) to breast tissue is an essential aspect of mammography equipment evaluations and exposure controls. The American College of Radiology (ACR) Quality Control Manual outlines the procedure for MGD determination in screen-film mammography based upon conversions of entrance skin exposures (ESEs) measured with an ionization chamber (IC). The development of digital mammography has increased with the demand for improved object resolution and tissue contrast. This change in image receptor from screen-film to a solid-state detector has led to questions about the applicability of the ACR MGD methodology to digital mammography. This research has validated the applicability of the ACR MGD methodology to digital mammography in the GE digital mammography system Senographe 2000D. MGD was determined using light output measurements from thermoluminescent dosimeters (MGDTL), exposure measurements from an IC (MGD(IC)) and conversion factors from the ACR Mammography Quality Control Manual. MGD(TL) and MGD(IC) data indicate that there is a statistically significant difference between the two measurements with the Senographe 2000D. However, the applicability of the ACR's methodology was validated by calculating MGD at various depths in a 50/50 breast phantom. Additionally, the results of backscatter measurements from the image receptors of both mammography modalities indicate there is a difference (all P values < 0.001) in the radiation backscattered from each image receptor.