Department of Imaging Physics, University of Texas MD Anderson Cancer Center, Houston, Texas 77030, USA.
Med Phys. 2011 Feb;38(2):589-97. doi: 10.1118/1.3521469.
In this article, we describe a method to estimate the spatial dose variation, average dose and mean glandular dose (MGD) for a real breast using Monte Carlo simulation based on cone beam breast computed tomography (CBBCT) images. We present and discuss the dose estimation results for 19 mastectomy breast specimens, 4 homogeneous breast models, 6 ellipsoidal phantoms, and 6 cylindrical phantoms.
To validate the Monte Carlo method for dose estimation in CBBCT, we compared the Monte Carlo dose estimates with the thermoluminescent dosimeter measurements at various radial positions in two polycarbonate cylinders (11- and 15-cm in diameter). Cone-beam computed tomography (CBCT) images of 19 mastectomy breast specimens, obtained with a bench-top experimental scanner, were segmented and used to construct 19 structured breast models. Monte Carlo simulation of CBBCT with these models was performed and used to estimate the point doses, average doses, and mean glandular doses for unit open air exposure at the iso-center. Mass based glandularity values were computed and used to investigate their effects on the average doses as well as the mean glandular doses. Average doses for 4 homogeneous breast models were estimated and compared to those of the corresponding structured breast models to investigate the effect of tissue structures. Average doses for ellipsoidal and cylindrical digital phantoms of identical diameter and height were also estimated for various glandularity values and compared with those for the structured breast models.
The absorbed dose maps for structured breast models show that doses in the glandular tissue were higher than those in the nearby adipose tissue. Estimated average doses for the homogeneous breast models were almost identical to those for the structured breast models (p=1). Normalized average doses estimated for the ellipsoidal phantoms were similar to those for the structured breast models (root mean square (rms) percentage difference = 1.7%; p = 0.01), whereas those for the cylindrical phantoms were significantly lower (rms percentage difference = 7.7%; p < 0.01). Normalized MGDs were found to decrease with increasing glandularity.
Our results indicate that it is sufficient to use homogeneous breast models derived from CBCT generated structured breast models to estimate the average dose. This investigation also shows that ellipsoidal digital phantoms of similar dimensions (diameter and height) and glandularity to actual breasts may be used to represent a real breast to estimate the average breast dose with Monte Carlo simulation. We have also successfully demonstrated the use of structured breast models to estimate the true MGDs and shown that the normalized MGDs decreased with the glandularity as previously reported by other researchers for CBBCT or mammography.
本文描述了一种使用基于锥形束乳腺 CT（CBBCT）图像的蒙特卡罗模拟来估算真实乳房的空间剂量变化、平均剂量和乳腺平均剂量（MGD）的方法。我们呈现并讨论了 19 个乳房切除术标本、4 个均匀乳房模型、6 个椭圆形体模和 6 个圆柱形体模的剂量估算结果。
为了验证蒙特卡罗方法在 CBBCT 中的剂量估算，我们在两个聚碳酸酯圆柱体内（直径分别为 11 和 15 厘米）的不同径向位置处，将蒙特卡罗剂量估计与热释光剂量计测量值进行了比较。使用台式实验扫描仪获得的 19 个乳房切除术标本的锥形束 CT（CBCT）图像被分割并用于构建 19 个结构乳房模型。使用这些模型进行 CBBCT 的蒙特卡罗模拟，并用于估算等中心单位开空气暴露的点剂量、平均剂量和乳腺平均剂量。计算了基于质量的乳腺密度值，并用于研究它们对平均剂量以及乳腺平均剂量的影响。还估算了 4 个均匀乳房模型的平均剂量，并将其与相应的结构乳房模型进行了比较，以研究组织结构的影响。对于具有相同直径和高度的椭圆形和圆柱形数字体模，还估算了各种乳腺密度值的平均剂量，并将其与结构乳房模型进行了比较。
结构乳房模型的吸收剂量图显示，腺体组织中的剂量高于附近的脂肪组织。对于均匀乳房模型的估计平均剂量几乎与结构乳房模型相同（p=1）。对于椭圆形体模的归一化平均剂量估计值与结构乳房模型相似（均方根（rms）百分比差异=1.7%；p=0.01），而对于圆柱形体模的归一化平均剂量则明显较低（rms 百分比差异=7.7%；p<0.01）。发现乳腺平均剂量随乳腺密度的增加而降低。
我们的结果表明，使用从 CBCT 生成的结构乳房模型得出的均匀乳房模型足以估算平均剂量。本研究还表明，类似尺寸（直径和高度）和乳腺密度的椭圆形数字体模可用于代表真实乳房，使用蒙特卡罗模拟估算平均乳房剂量。我们还成功地使用结构乳房模型来估算真实的 MGD，并表明与其他研究人员之前报道的 CBBCT 或乳房 X 光检查相比，归一化 MGD 随着乳腺密度的增加而降低。
