SU‐C‐141‐06: Registration Accuracy with Four‐Dimensional Cone‐Beam CT for Lung Cancer Treatment

Abstract

Purpose: A four‐dimensional (4D) cone‐beam CT (CBCT) is widely implemented in the image‐guided radiation therapy (IGRT) for lung cancer treatment. Recently, we established a verifying system using in‐treatment 4D CBCT acquired during Volumetric Modulated Arc Therapy (VMAT). With this system, we assess the registration accuracy with pre‐treatment 4D CBCT for IGRT. Methods: Fifteen lung cancer patients treated with single‐arc VMAT were selected in this study. The breathing condition of all patients was constrained with a stereotactic body frame (ELEKTA) accompanied by abdominal compression plate. The gross tumor volumes were contoured on 10 CT datasets from 4D CT (TOSHIBA and ANZAI) and merged into internal target volumes (ITVs). The planning target volume (PTV) margin was isotropically taken as 5 mm. The patient registration was performed by an Elekta x‐ray volume imaging functionality version 4.5 (XVI ver.4.5), which allows to use 4D CBCT respiratory sorting for registration. The VMAT treatment was performed with simultaneous acquisition of the kV projection images. The in‐treatment 4D CBCT was reconstructed by in‐house software. The reconstructed images were exported to a treatment planning system. To assess set‐up errors and respiratory motion, the displacement of tumors between the planning ITV and the shadow of the tumor in in‐treatment 4D CBCT images was measured in CC, left‐right (LR), and anterior‐posterior (AP) directions. Results: The average displacements between the ITV and the actual tumor location during treatment were 0.24 +/− 0.45, 0.19 +/− 0.46, and 0.42 +/− 0.43 mm for the CC, LR, and AP directions, respectively. The discrepancy in each phase did not exceed 3 mm in any direction. Conclusion: With in‐treatment 4D CBCT, which is a direct method for quantitatively assessing the intrafractional location, we confirmed that the required PTV margins were 3 mm when the registration was performed using pre‐treatment 4D CBCT. This work was supported by JSPS KAKENHI 22791176.