Rivard Mark J, Ghadyani Hamid R, Bastien Adam D, Lutz Nicholas N, Hepel Jaroslaw T
Department of Radiation Oncology, Tufts University School of Medicine, Boston, MA.
Department of Engineering, Farmingdale State College SUNY, Farmingdale, NY.
J Contemp Brachytherapy. 2015 Feb;7(1):55-71. doi: 10.5114/jcb.2015.49355. Epub 2015 Feb 17.
Noninvasive image-guided breast brachytherapy delivers conformal HDR (192)Ir brachytherapy treatments with the breast compressed, and treated in the cranial-caudal and medial-lateral directions. This technique subjects breast tissue to extreme deformations not observed for other disease sites. Given that, commercially-available software for deformable image registration cannot accurately co-register image sets obtained in these two states, a finite element analysis based on a biomechanical model was developed to deform dose distributions for each compression circumstance for dose summation.
The model assumed the breast was under planar stress with values of 30 kPa for Young's modulus and 0.3 for Poisson's ratio. Dose distributions from round and skin-dose optimized applicators in cranial-caudal and medial-lateral compressions were deformed using 0.1 cm planar resolution. Dose distributions, skin doses, and dose-volume histograms were generated. Results were examined as a function of breast thickness, applicator size, target size, and offset distance from the center.
Over the range of examined thicknesses, target size increased several millimeters as compression thickness decreased. This trend increased with increasing offset distances. Applicator size minimally affected target coverage, until applicator size was less than the compressed target size. In all cases, with an applicator larger or equal to the compressed target size, > 90% of the target covered by > 90% of the prescription dose. In all cases, dose coverage became less uniform as offset distance increased and average dose increased. This effect was more pronounced for smaller target-applicator combinations.
The model exhibited skin dose trends that matched MC-generated benchmarking results within 2% and clinical observations over a similar range of breast thicknesses and target sizes. The model provided quantitative insight on dosimetric treatment variables over a range of clinical circumstances. These findings highlight the need for careful target localization and accurate identification of compression thickness and target offset.
非侵入性图像引导乳腺近距离放射治疗在乳腺受压的情况下进行适形高剂量率（192）铱近距离放射治疗，治疗方向为头脚方向和内外方向。该技术使乳腺组织承受其他疾病部位未观察到的极端变形。鉴于此，用于可变形图像配准的商业软件无法准确地对在这两种状态下获得的图像集进行配准，因此开发了一种基于生物力学模型的有限元分析方法，以针对每种压缩情况对剂量分布进行变形，以便进行剂量求和。
该模型假设乳腺处于平面应力状态，杨氏模量值为30 kPa，泊松比为0.3。使用0.1 cm的平面分辨率对头脚方向和内外方向压缩时圆形及皮肤剂量优化施源器的剂量分布进行变形。生成剂量分布、皮肤剂量和剂量体积直方图。结果作为乳腺厚度、施源器尺寸、靶区尺寸以及距中心的偏移距离的函数进行研究。
在所研究的厚度范围内，随着压缩厚度的减小，靶区尺寸增加了几毫米。这种趋势随着偏移距离的增加而增强。施源器尺寸对靶区覆盖的影响最小，直到施源器尺寸小于压缩后的靶区尺寸。在所有情况下，当施源器尺寸大于或等于压缩后的靶区尺寸时 > 90% 的靶区被 > 90% 的处方剂量覆盖。在所有情况下，随着偏移距离增加和平均剂量增加，剂量覆盖变得不那么均匀。对于较小的靶区 - 施源器组合，这种影响更为明显。
该模型显示的皮肤剂量趋势与蒙特卡罗生成的基准结果在2% 以内匹配，并且在类似的乳腺厚度和靶区尺寸范围内与临床观察结果相符。该模型在一系列临床情况下提供了关于剂量学治疗变量的定量见解。这些发现突出了仔细进行靶区定位以及准确识别压缩厚度和靶区偏移的必要性。
