對乳房攝影之平均乳腺劑量發展簡化的計算方法

前言評估平均乳腺劑量(Average glandular dose, AGD)為乳房攝影的重要議題，在評估AGD前必須先知道乳房壓迫厚度(Compressed Breast Thickness, CBT)與乳腺含量百分率(Percentage Glandular Content, PGC)，因此計算過程繁複。本研究的目的為發展簡化的等效厚度乳房模式(Equivalent Thickness Breast Model, ET-BM)之方法評估乳房攝影之AGD。材料與方法本研究首先以等效厚度觀念建立ET-BM之AGD評估方法；其次是使用Siemens (Mammomat NovationDR)與Hologic (Selenia Dimensions)乳房攝影系統，採用不同厚度(2~6 cm)及不同材質(PGC = 0~100%)之硬質假體進行實驗；接著是應用ET-BM之方法計算不同可壓迫假體於全景式數位乳房攝影(Full Field Digital Mammography, FFDM)攝影之AGD，最後是應用ET-BM之方法計算可壓迫假體於數位式乳房斷層攝影(Digital Breast Tomosynthesis, DBT)攝影之AGD。結果本研究應用ET-BM方法計算硬質假體之AGD，結果顯示其AGD的誤差範圍為-0.07~0.25 mGy；在計算可壓迫假體於FFDM攝影之AGD，其均方根誤差(Root-Mean-Square Errors, RMSE)為0.03~0.07 mGy；在計算可壓迫假體於DBT攝影之AGD的RMSE為0.08 mGy。結論本研究ET-BM之AGD評估方法可以不需要知道CBT與PGC，只需要使用臨床攝影條件即可評估AGD。由實驗結果可知其計算AGD時具有相當高之準確度，因此非常適用在快速地評估乳房攝影之AGD。

IntroductionEstimation of average glandular dose (AGD) is an important issue in mammography. The AGD calculation method is complicate due to the fact that the compressed breast thickness (CBT) and percentage glandular content (PGC) have to be known. The purpose of this study is to develop the simplified AGD calculation method using equivalent thickness breast model (ET-BM).Material and methodsThe first step in this study is the development of simplified AGD calculation method using ET-BM. The second step is the verification of the ET-BM AGD calculation method using rigid phantoms with different thicknesses (2~6 cm) and compositions (PGC=0~100%). The third step is the estimation of the AGD of different compressible breast phantoms with the ET-BM AGD calculation method in Full field digital mammography (FFDM) procedure. The last step is the estimation of the AGD of different compressible breast phantoms with the ET-BM AGD calculation method in Digital breast tomosynthesis (DBT) procedure.ResultsResults from this study show that the error of AGD of rigid phantoms calculated using the ET-BM AGD calculation method ranging from -0.07~0.25 mGy. The values of root-mean-square errors (RMSE) on AGD for different compressible breast phantoms are 0.03~0.07 mGy in different FFDM systems. The value of RMSE on AGD for different compressible breast phantoms is 0.08 mGy in DBT procedure.ConclusionThe ET-BM AGD calculation method proposed in this study can estimate AGD using imaging factors while without information about CBT and PGC in mammography. By applying the ET-BM AGD calculation method, the accuracy of AGD is acceptable in clinical practice. This method is suitable for rapidly AGD estimation in mammography.