Real-time online matching in high dose treatments : Do RTTs perform as well as physicians?

Abstract

Purpose or Objective In routine practice of modern radiotherapy, cancer patients are scanned with a computer tomography (CT) scanner to obtain a set of CT images (planning CT) for treatment planning. Before treatment delivery, the patient position is verified by using Cone Beam Computed Tomography (CBCT) or conventional orthogonal planar image (OPI), for matching with the planning CT or the digitally reconstructed radiographs (DRR) generated from the planning CT respectively. This study aims at evaluating the dosimetric impact and the matching time of using CBCT and OPI in position verification in radiotherapy of prostate cancer. Material and Methods Fifteen prostate cancer patients positioned with CBCT during radiotherapy were recruited retrospectively. OPI were simulated by generating DRR using CBCT in Eclipse™ treatment planning system (Varian Medical Systems, Palo Alto, CA). 3D-3D matching on CBCT/planning CT and 2D-2D matching on simulated OPI/DRR were performed in MIM Maestro™ (MIMSoftware, inc., Cleveland, OH, USA). Time spent on matching was recorded. Treatment plans were created on CBCT and the matching results were applied for dose calculation in Eclipse™.The two position verification methods were compared in terms of Isodisplacement vector (IDV), conformity index and homogeneity index of targets, dose-volume parameters of bladder and rectum in the resultant dose distributions, and matching time consumption. The results were tested using two-tailed Wilcoxon matched pairs signed rank test with a significance level of 0.005. Results Largest differences in IDV of CBCT-based and OPI-based position verification were found in antero-posterior direction (average 1.6 mm) and were statistically significant. The conformity index and homogeneity index of targets, the dose-volume parameters of bladder and rectum of the two position verification methods are summarized in Table 1. The use of CBCT resulted in a better conformity and homogeneity of the targets. Dosimetrically, CBCT was superior than OPI in terms of bladder dose but slightly inferior than OPI in terms of rectum dose for position verification. The scatter plot for matching time consumption in CBCT-based and OPI-based position verification in each fractions are shown in Figure 1. The time spent on performing 3D-3D matching and 2D-2D matching were 4.2 ± 0.5 minutes and 1.7 ± 0.3 minutes respectively and the differences were statistically significant. Conclusion CBCT-based position verification yields a significant different IDV and is dosimetrically beneficial comparing with OPI-based position verification in radiotherapy treatment of prostate cancer. However, in addition to CBCT acquisition time, CBCT-based position verification requires a longer matching time than OPI-based position verification. Therefore, the choice of position verification methods should depend on the availability of resources in the radiotherapy department and the tolerance on the treatment accuracy.