Master of Science (MS)
Physics and Astronomy
Purpose/Objectives: A drawbackof tandem and ovoid (T&O) ICBT is exposure of the posterior bladder and anterior rectal walls to relatively high isodoses. To mitigate this effect, intra-ovoid shielding may be used to reducedose to these OARs. However, metal artifactspresent in images acquired via kVCT make anatomy segmentation and catheter localization difficult for the purpose of 3D treatment planning. We present a method that combines MVCT-based imaging and applicator modeling to increase the quality of 3D treatment plans for shielded T&O ICBT. Materials/Methods: Using Oncentra’s TPS, 9 participants from multiple institutions performed organ segmentation and catheter reconstruction for KVCT and MVCT data sets acquired of a water phantom containing bladder and rectum surrogatesand various HDR T&O applicators: Nucletron’s CT/MR compatible(CTMR),Nucletron’s shielded Fletcher Williamson (FW) and (3) Varian’s shielded Fletcher-Suit-Delclos style(FSD).The dimensions of OAR structures were determined using in-air kVCT and physical measurements. By comparing the 3D volumes andcentroid-to-perimeter (C2P) measurements of individual OAR contours, segmentation accuracy was assessed in regions exhibiting artifact under kVCT (1cm superior and inferior to shielding). Comparing the TPS-defined coordinate of the most distal dwell position to that of the true position (determined using radiographs of a fiducial affixed to the applicators), assessed catheter reconstruction accuracy. For Nucletron devices, this metricwas also quantified using an applicator-model for localization. Results:The percentage of points for C2P measurements that differ (greater than 2mm) from the true contour extentsdecreased underMVCT for the shielded T&Os (78.4 vs. 71.3%), while the converse is observed for the CTMR. Similarly, the volume of the OAR surrogates follows the same trend. This is attributed to the lack of metal artifacts as well as the decrease in the contrast of low Z materials observed when utilizing MVCT. Catheter reconstruction accuracy improved by 26% under MVCT for shielded T&Os, was invariant for the CTMR and within 2.29mm of the true position using applicator modeling. Conclusions:The quality of MVCT 3D ICBT treatment plans of shielded T&O is comparable to MVCT CTMR treatment plans. Further improvements were observed when using an applicator model for catheter reconstruction.
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Kemp, Jeffrey Roger, "The feasibility of using megavoltage CT for the treatment planning of HDR cervical brachytherapy with shielded tandem and ovoid applicators" (2012). LSU Master's Theses. 970.
Price, Michael J