- 學位論文
評估全景式乳房攝影與數位式乳房斷層攝影之劑量分布
Measurement of dose distributions for full-field digital mammography and digital breast tomosynthesis
摘要
前言 乳房攝影為臨床上偵測乳癌的一項重要檢查，近年來常使用全景式乳房攝影(full-field digital mammography, FFDM)與數位式乳房斷層攝影(digital breast tomosynthesis, DBT)技術進行乳篩。目前臨床上常用之乳房假體多為剛體結構(rigid construction)，其並不能呈現乳房壓迫時所導致的形變。先前的研究已對Bolus假體之厚度量測與影像品質進行探討，而本研究之目的為評估Bolus軟性材質乳房假體經全景式乳房攝影與數位式乳房斷層攝影之劑量分布情形。 材料與方法 本研究以層狀Bolus假體作為可壓迫假體進行實驗，以兩種攝影系統(Siemens Novation DR and Hologic Selenia Dimensions)進行攝影。在劑量部分，我們使用TLD-100H (LiF：Mg, Cu, P) 進行量測，並以Harshaw 3500型 TLD計讀儀器進行計讀。在空間與環境劑量方面，將TLD-100H黏貼於攝影空間中離地面90、120、140 cm處，分別模擬性腺、乳房與甲狀腺位置。在探討Bolus假體劑量方面，將TLD置於Bolus假體(2-8 cm)表面與不同深度間，使用FFDM與DBT系統搭配不同靶/濾片組合(Mo/Mo、Mo/Rh、W/Rh、W/Ag、W/Al)進行攝影，以評估回散射因子(backscatter factor, BSF)與深部劑量分布。隨後評估每次攝影條件計算之AGD與不同厚度Bolus假體中央層量測劑量，求得AGD之轉換因子，其定義為Bolus中央層量測劑量與AGD之比值。在臨床應用部分，將TLD置於不同厚度之Bolus假體(2、4、6、8 cm)中央層，使用固定式與彈性式壓迫板搭配不同壓迫力(78.3, 117.4與156.6 N)，以自動曝露控制模式(auto exposure control mode)進行攝影，並使用先前求得之AGD轉換因子評估Bolus假體之AGD。 結果與討論 在空間劑量分布方面，結果顯示在乳房位置之量測劑量高於甲狀腺與性腺位置之劑量。在環境劑量分布方面，左右側牆壁之量測劑量(0.828-1.650 mGy)高於前後側牆壁之量測劑量(0.001-0.408 mGy)。BSF方面，Bolus假體之BSF(1.082〜1.122 mGy/mGy)與目前常用乳房假體材質(PMMA，BR-12，BR-fat)的BSF(1.006-1.102 mGy/mGy)非常接近，且在歐洲規範之建議值範圍內(1.07-1.13 mGy/mGy)。在深部劑量方面，不同厚度Bolus假體(2-8 cm)之量測劑量隨假體厚度增加而下降。以FFDM模式攝影時，Bolus假體中央層之量測劑量分別為0.51±0.01、0.65±0.01、0.70±0.01、1.39±0.001、2.02±0.04、1.95±0.03與1.99±0.05 mGy；以Tomo模式攝影時，中央層之量測劑量分別為1.20±0.02、1.22±0.04、1.55±0.03、1.78±0.001、2.38±0.03、3.10±0.03以及3.63±0.02 mGy。結果顯示以Tomo模式攝影時，Bolus假體接收之AGD較FFDM模式攝影之AGD高。在AGD轉換因子方面，結果顯示以FFDM模式攝影時，AGD轉換因子隨Bolus假體厚度增加而上升(0.89-1.18 mGy/mGy)；但以Tomo模式攝影時，AGD轉換因子則隨Bolus假體厚度增加而下降(0.78-0.90 mGy/mGy)。而在臨床應用方面，結果呈現TLD量測劑量(1.24-3.09 mGy)高於計算AGD(1.06-2.93 mGy)。使用彈性式與固定式壓迫板攝影時，AGD預測值與計算值之平均誤差分別為-8±7 %與4±12 %。 結論 在空間劑量分布方面，乳房位置之散射劑量高於甲狀腺與性腺位置。而在環境劑量分布方面，左右方向具有較高之散射劑量。在BSF部分，結果顯示Bolus假體之BSF與常用之硬質假體相似，此結果代表Bolus假體適合用於評估乳房攝影劑量。在AGD轉換係數方面，結果表示能夠以Bolus假體中央層之量測劑量評估AGD。在臨床應用中，TLD並沒有在壓迫的過程中損壞。由此可知，Bolus假體搭配TLD適合在臨床乳房攝影壓迫過程中用於劑量分布之量測。
並列摘要
Introduction X-ray mammography is an important clinical examination for breast cancer detection. At present, full-field digital mammography and digital breast tomosynthesis are frequently used imaging techniques. The currently used breast phantoms are rigid construction and cannot be deformed during the compression procedure. In our previous study, Bolus phantom was introduced for thickness measurement and image quality assessment. The purpose of this study is the measurement of dose distributions with Bolus phantom for full-field digital mammography and digital breast tomosynthesis. Material and methods In this study, Bolus slabs were used to design the compressible phantom. Two mammography systems, Siemens Novation DR and Hologic Selenia Dimensions, were used. Total of 210 TLD-100H (LiF：Mg, Cu, P) chips and the Harshaw 3500 TLD reader were used for dose measurement. For spatial and environmental dose measurements, several TLD-100H chips were placed at the location of 90, 120 and 140 cm above the ground to simulate gonad, breast and thyroid positions, respectively. For dose assessment with Bolus phantom, backscatter factor (BSF) and depth dose were measured for target/filter combination of Mo/Mo, Mo/Rh, W/Rh, W/Ag and W/Al. TLD chips was placed on the surface and embedded in different depths of Bolus phantom (2-8 cm). These Bolus phantoms were irradiated with FFDM and DBT systems. The BSF and depth dose of Bolus phantom were calculated. AGD of Bolus phantom for each exposure was estimated. The conversion factor of AGD, defined as the ratio of AGD and measured dose at central layer of Bolus phantom, was calculated for each thickness of Bolus phantom. For clinical application, the TLD-100H chips were embedded in the central layer of Bolus phantoms (2, 4, 6, 8 cm). These Bolus phantoms were compressed with different forces (78.3, 117.4 and 156.6 N) and irradiated with auto exposure control mode. The conversion factors of AGD were applied to estimate the AGD of Bolus phantoms. Results and discussion For spatial dose distribution, the measurement doses of breast position are greater than thyroid and gonad positions. For environmental dose distribution, the range of measured doses of right and left wall was 0.828-1.650 mGy which is greater than the measured doses of anterior and posterior wall (0.001-0.408 mGy). For the BSF assessment, the BSF of Bolus phantom is ranging from 1.082 to 1.122 mGy/mGy. The BSF values of Bolus phantom obtained are comparable to those of currently used breast phantom (1.006-1.102 mGy/mGy) and are within the suggested range of the European guideline (1.07-1.13 mGy/mGy). For the depth dose of Bolus phantom, the dose measured by TLD decreased with increasing depth of Bolus phantom for both of FFDM and Tomo mode. For FFDM mode, the measured doses at the central layer were 0.51±0.01, 0.65±0.01, 0.70±0.01, 1.39±0.001, 2.02±0.04, 1.95±0.03, and 1.99±0.05 mGy for 2-, 3-, 4-, 5-, 6-, 7-, and 8-cm Bolus phantom, respectively. For Tomo mode, the measured doses at the central layer were 1.20±0.02, 1.22±0.04, 1.55±0.03, 1.78±0.001, 2.38±0.03, 3.10±0.03, and 3.63±0.02 mGy for 2-, 3-, 4-, 5-, 6-, 7-, and 8-cm Bolus phantom, respectively. For each exposure, the AGD of Bolus phantom imaging with Tomo mode was higher than AGD of Bolus phantom imaging with FFDM mode. In this study, the conversion factor of AGD increased with increasing of Bolus phantom for FFDM mode (0.89-1.18 mGy/mGy) but the conversion factor of AGD varied slightly for Tomo mode (0.78-0.90 mGy/mGy). For clinical application, the measured doses by TLD (1.24-3.09 mGy) were greater than the calculated AGD (1.06-2.93 mGy). The average prediction errors on AGD were -8±7 % and 4±12 % for flexible and rigid paddle, respectively. Conclusion For spatial and environmental dose measurement, the measured spatial doses of breast position are higher than those of thyroid and gonad positions, and the measured environmental doses of left and right directions are higher than anterior and posterior directions. In this study, the BSF of Bolus phantom is similar to those of frequently used phantom materials in mammography. Therefore, Bolus phantom is suitable for dose assessment in mammography. Applying the conversion factor of AGD purposed in this study, the AGD of Bolus phantom can be estimated by measuring the doses at central layer of Bolus phantom. In clinical application, the TLD chips did not break during the compression procedure. Therefore, the Bolus phantom combined TLD chip is suitable for the measurement of dose distribution during the clinical compression procedure in mammography.