Author: Sarah Burleson, Antonio L. Damato, Yabo Fu, Laura Happersett, Tianfang Li, Xiang Li, Himanshu Nagar, Pengpeng Zhang 👨🔬
Affiliation: Department of Medical Physics, Memorial Sloan Kettering Cancer Center, Memorial Sloan Kettering Cancer Center 🌍
Purpose: An iodinated hydrogel perirectal spacer, injected between the prostate and rectum, reduces rectal high-dose exposure during prostate SBRT. We assessed the feasibility of tracking the spacer alongside prostate fiducials for detecting prostate deformation and rectal motion, ensuring accurate tumor coverage and rectum sparing during gated VMAT delivery.
Materials/Methods: Although fully visualized on planning CT, the spacer is obscured by surrounding structures on kV images from many projection angles during VMAT delivery. We applied a novel technique of decomposition tissue imaging (DTI) to improve the spacer’s detectibility. The spacer was first segmented from the planning CT and prior CBCTs. Pairs of regular DRR and spacer-only DRR were generated and used to train a conditional generative adversarial network to enhance the spacer’s visibility on kV images. During delivery, the on-treatment kV images were converted to spacer-enhanced images. A template-matching algorithm localized spacer and fiducials in real time by registering the on-treatment images to corresponding templates created from the planning CT. The spacer displacement from DTI-driven tracking was compared with the 3D rigid registration of planning CT and mid-treatment CBCT to evaluate the tracking accuracy.
Results: In a retrospective study of 10 prostate patients, the average prostate deformation, measured by relative motion between fiducial and spacer centroids, was 2.2±1.3 mm. The institutional 3-mm margin around the prostate was appropriate for tumor coverage. DTI improved the rate of spacer detection to above 90% from 40% with raw kV images. Tracking accuracy was <1 mm, aligning with the 1.5-mm action threshold for gated delivery.
Conclusion: DTI enables robust tracking of the rectal spacer, allowing precise prostate and rectum localization during treatment. The comprehensive motion data support revising intrafractional management strategies, such as setting structure-specific gating thresholds to address prostate and rectum motions, to enhance treatment accuracy and efficacy.