Yao Weiguang, Zhang Baoshe, Han Dong, Polf Jerimy, Vedam Sastry, Lasio Giovanni, Yi Byongyong
Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, Maryland, USA.
Med Phys. 2022 Nov;49(11):6794-6801. doi: 10.1002/mp.15915. Epub 2022 Aug 15.
Anatomic variation has a significant dosimetric impact in intensity-modulated proton therapy. Weekly or biweekly computed tomography (CT) scans, called quality assurance CTs (QACTs), are used to monitor anatomic and resultant dose changes to determine whether adaptive plans are needed. Frequent CT scans result in unwanted QACT dose and increased clinical workloads. This study proposed utilizing patient setup cone-beam CTs (CBCTs) and treatment plan robustness to reduce the frequency of QACTs.
We retrospectively analyzed data from 27 patients with head-and-neck cancer, including 594 CBCTs, 136 QACTs, and 19 adaptive plans. For each CBCT, water-equivalent thickness (WET) along the pencil-beam path was calculated. For each treatment plan, the WET of the first-day CBCT was used as the reference, and the mean WET changes (ΔWET) in each following CBCT was used as the surrogate of proton range change. Using CBCTs acquired prior to a QACT, we predicted the ΔWET on the QACT day by a linear regression model. The impact of range change on target dose was calculated as the predicted ΔWET weighted by the monitor units of each field. In addition, plan robustness was estimated from the robust dose-volume histograms (DVHs) and combined with ΔWET to reduce QACT frequency. Robustness was estimated from the distance between the DVH curves of the nominal and worst scenarios.
When the estimated mean ΔWET was <6.5 mm (or <7.5 mm if the robustness was >95%), the QACT could be skipped without missing any adaptive planning; otherwise a QACT was required. Overall, 41% of QACTs could be eliminated when ΔWET was <6.5 mm and 56% when ΔWET was <7.5 mm, and robustness was >95%. At least one QACT could have been omitted in 25 of the 27 cases under skipping thresholds at ΔWETs <7.5 mm and R > 95%.
This study suggests that the number of QACTs can be greatly reduced by calculating range change in patient setup CBCTs and can be further reduced by combining this information with analyses of plan robustness.
解剖变异在调强质子治疗中对剂量学有显著影响。每周或每两周进行的计算机断层扫描（CT），即质量保证CT（QACT），用于监测解剖结构及由此产生的剂量变化，以确定是否需要适应性计划。频繁的CT扫描会导致不必要的QACT剂量，并增加临床工作量。本研究提出利用患者摆位锥形束CT（CBCT）和治疗计划稳健性来减少QACT的频率。
我们回顾性分析了27例头颈癌患者的数据，包括594次CBCT、136次QACT和19个适应性计划。对于每次CBCT，计算铅笔束路径上的水等效厚度（WET）。对于每个治疗计划，将第一天CBCT的WET用作参考，后续每次CBCT的平均WET变化（ΔWET）用作质子射程变化的替代指标。利用在QACT之前获取的CBCT，通过线性回归模型预测QACT当天的ΔWET。射程变化对靶区剂量的影响通过每个射野的监测单位加权预测的ΔWET来计算。此外，从稳健剂量体积直方图（DVH）估计计划稳健性，并将其与ΔWET相结合以降低QACT频率。稳健性通过标称和最差情况的DVH曲线之间的距离来估计。
当估计的平均ΔWET <6.5毫米（如果稳健性>95%，则<7.5毫米）时，可以跳过QACT而不会错过任何适应性计划；否则需要进行QACT。总体而言，当ΔWET <6.5毫米时，41%的QACT可以省略；当ΔWET <7.5毫米且稳健性>95%时，56%的QACT可以省略。在27例病例中的25例中，在ΔWET <7.5毫米且稳健性>95%的跳过阈值下，至少可以省略一次QACT。
本研究表明，通过计算患者摆位CBCT中的射程变化，可以大幅减少QACT的数量，并且通过将此信息与计划稳健性分析相结合，可以进一步减少QACT的数量。
