Ray Xenia, Bojechko Casey, Moore Kevin L
Department of Radiation Medicine and Applied Sciences, UCSD Moores Cancer Center, La Jolla, CA, USA.
J Appl Clin Med Phys. 2019 Nov;20(11):131-143. doi: 10.1002/acm2.12749. Epub 2019 Oct 6.
The Varian Halcyon™ electronic portal imaging detector is always in-line with the beam and automatically acquires transit images for every patient with full-field coverage. These images could be used for "every patient, every monitor unit" quality assurance (QA) and eventually adaptive radiotherapy. This study evaluated the imager's sensitivity to potential clinical errors and day-to-day variations from clinical exit images.
Open and modulated fields were delivered for each potential error. To evaluate output changes, monitor units were scaled by 2%-10% and delivered to solid water slabs and a homogeneous CIRS phantom. To mimic weight changes, 0.5-5.0 cm of buildup was added to the solid water. To evaluate positioning changes, a homogeneous and heterogeneous CIRS phantom were shifted 2-10 cm and 0.2-1.5 cm, respectively. For each test, mean relative differences (MRDs) and standard deviations in the pixel-difference histograms (σ ) between test and baseline images were calculated. Lateral shift magnitudes were calculated using cross-correlation and edge-detection filtration. To assess patient variations, MRD and σ were calculated from six prostate patients' daily exit images and compared between fractions with and without gas present.
MRDs responded linearly to output and buildup changes with a standard deviation of 0.3%, implying a 1% output change and 0.2 cm changes in buildup could be detected with 2.5σ confidence. Shifting the homogenous phantom laterally resulted in detectable MRD and σ changes, and the cross-correlation function calculated the shift to within 0.5 mm for the heterogeneous phantom. MRD and σ values were significantly associated with the presence of gas for five of the six patients.
Rapid analyses of automatically acquired Halcyon™ exit images could detect mid-treatment changes with high sensitivity, though appropriate thresholds will need to be set. This study presents the first steps toward developing effortless image evaluation for all aspects of every patient's treatment.
瓦里安Halcyon™电子门静脉成像探测器始终与射束对齐，并自动为每位患者获取全视野覆盖的传输图像。这些图像可用于“每位患者，每个监测单位”的质量保证（QA），并最终用于自适应放射治疗。本研究评估了该成像仪对潜在临床误差和临床出射图像日常变化的敏感性。
针对每种潜在误差提供开放野和调制野。为评估输出变化，将监测单位按2%-10%缩放，并输送至固体水模体和均匀的CIRS体模。为模拟体重变化，在固体水上添加0.5-5.0厘米的建成区。为评估定位变化，将均匀和非均匀的CIRS体模分别平移2-10厘米和0.2-1.5厘米。对于每项测试，计算测试图像与基线图像之间像素差异直方图中的平均相对差异（MRD）和标准差（σ）。使用互相关和边缘检测滤波计算横向偏移量。为评估患者差异，从六名前列腺癌患者的每日出射图像中计算MRD和σ，并比较有气和无气分次之间的差异。
MRD对输出和建成区变化呈线性响应，标准差为0.3%，这意味着在2.5σ置信度下可检测到1%的输出变化和0.2厘米的建成区变化。横向移动均匀体模会导致可检测到的MRD和σ变化，互相关函数计算出非均匀体模的偏移量在0.5毫米以内。六名患者中有五名患者的MRD和σ值与气体的存在显著相关。
对自动获取的Halcyon™出射图像进行快速分析可高灵敏度地检测治疗过程中的变化，不过需要设置适当的阈值。本研究为开展针对每位患者治疗各方面的轻松图像评估迈出了第一步。
