Olch Arthur J, Alaei Parham
Radiation Oncology Program, Children's Hospital Los Angeles, Los Angeles, CA, USA.
Department of Radiation Oncology, University of Minnesota, Minneapolis, MN, USA.
J Appl Clin Med Phys. 2021 Feb;22(2):85-89. doi: 10.1002/acm2.13164. Epub 2021 Jan 15.
Cone beam computed tomography (CBCT) is often used for patient setup based solely on bony anatomy. The goal of this work was to evaluate whether CBCT dose can be lowered to the level of kV image pair doses when used for bony anatomy-based IGRT without compromising positioning accuracy.
An anthropomorphic phantom was CT scanned in the head, head and neck, chest, and pelvis regions and setup on the linear accelerator couch with the isocenter near the planned location. Cone beam computed tomographies were performed with the standard "full dose" protocol supplied by the linac vendor. With sequentially lowering the dose, three-dimensional (3D) matching was performed for each without shifting the couch. The standard kV image pair protocol for each site was also used to image the phantoms. For all studies, six degrees of freedom was included in the 2D or 3D matching to the extent they could be employed. Imaging doses were determined in air at isocenter following the TG-61 formalism.
Cone beam computed tomography dose was reduced by 81-98% of the standard CBCT protocol to nearly that of the standard kV image pair dose for each site. Relative to the standard CBCT shift values, translational shifts were within 0.3 and 1.6 mm for all sites, for the reduced dose CBCT and kV image pair, respectively. Rotational shifts were within 0.2 degree and 0.7 degrees for all sites, for the reduced dose CBCTs and kV image pair, respectively.
For bony anatomy-based image guidance, CBCT dose can be reduced to a value similar to that of a kV image pair with similar or better patient positioning accuracy than kV image pair alignment. Where rotations are important to correct, CBCT will be superior to orthogonal kV imaging without significantly increased imaging dose. This is especially important for image guidance for pediatric patient treatments.
锥形束计算机断层扫描（CBCT）常用于仅基于骨骼解剖结构的患者摆位。本研究的目的是评估在不影响定位准确性的情况下，用于基于骨骼解剖结构的图像引导放射治疗（IGRT）时，CBCT剂量是否可降低至千伏（kV）图像对剂量的水平。
使用人体模型在头部、头颈部、胸部和骨盆区域进行CT扫描，并将其放置在直线加速器治疗床上，等中心靠近计划位置。使用直线加速器供应商提供的标准“全剂量”方案进行锥形束计算机断层扫描。随着剂量依次降低，在不移动治疗床的情况下对每个剂量进行三维（3D）匹配。每个部位的标准kV图像对方案也用于对人体模型进行成像。对于所有研究，在二维或三维匹配中尽可能纳入六个自由度。按照TG - 61形式主义在等中心的空气中确定成像剂量。
每个部位的锥形束计算机断层扫描剂量相对于标准CBCT方案降低了81% - 98%，几乎达到标准kV图像对剂量的水平。相对于标准CBCT的移位值，对于降低剂量的CBCT和kV图像对，所有部位的平移移位分别在0.3毫米和1.6毫米以内。对于降低剂量的CBCT和kV图像对，所有部位的旋转移位分别在0.2度和0.7度以内。
对于基于骨骼解剖结构的图像引导，CBCT剂量可降低至与kV图像对相似的值，且患者定位准确性与kV图像对对齐相似或更好。在旋转对于校正很重要的情况下，CBCT将优于正交kV成像，且不会显著增加成像剂量。这对于儿科患者治疗的图像引导尤为重要。
