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Original Article

Korean Journal of Medical Physics 2014; 25(1): 37-45

Published online March 30, 2014

Copyright © Korean Society of Medical Physics.

Study of Scatter Influence of kV-Conebeam CT Based Calculation for Pelvic Radiotherapy

골반 방사선 치료에서 산란이 kV-Conebeam CT 영상 기반의 선량계산에 미치는 영향에 대한 연구

KyoungJun Yoon*, Jungwon Kwak*, Byungchul Cho*, YoungSeok Kim*,SangWook Lee*, SeungDo Ahn*, SangHee Nam

윤경준*ㆍ곽정원*ㆍ조병철*ㆍ김영석*ㆍ이상욱*ㆍ안승도*ㆍ남상희

*Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of Medicine, Department of Biomedical Engineering, University of Biomedical Science and Engineering, Seoul, Korea

*울산대학교 의과대학 서울아산병원 방사선종양학과, 인제대학교 의생명공학대학 의용공학과

Received: February 24, 2014; Accepted: March 16, 2014

Abstract

The accuracy and uniformity of CT numbers are the main causes of radiation dose calculation error. Especially, for the dose calculation based on kV-Cone Beam Computed Tomography (CBCT) image, the scatter affecting the CT number is known to be quite different by the object sizes, densities, exposure conditions, and so on. In this study, the scatter impact on the CBCT based dose calculation was evaluated to provide the optimal condition minimizing the error. The CBCT images was acquired under three scatter conditions ("Under-scatter", "Over-scatter", and "Full-scatter") by adjusting amount of scatter materials around a electron density phantom (CIRS062, Tissue Simulation Technology, Norfolk, VA, USA). The CT number uniformities of CBCT images for water-equivalent materials of the phantom were assessed, and the location dependency, either “inner” or “outer” parts of the phantom, was also evaluated. The electron density correction curves were derived from CBCT images of the electron density phantom in each scatter condition. The electron density correction curves were applied to calculate the CBCT based doses, which were compared with the dose based on Fan Beam Computed Tomography (FBCT). Also, 5 prostate IMRT cases were enrolled to assess the accuracy of dose based on CBCT images using gamma index analysis and relative dose differences. As the CT number histogram of phantom CBCT images for water equivalent materials was fitted with a gaussian function, the FHWM (146 HU) for "Full-scatter" condition was the smallest among the FHWM for the three conditions (685 HU for "under scatter" and 264 HU for "over scatter"). Also, the variance of CT numbers was the smallest for the same ingredients located in the center and periphery of the phantom in the "Full-scatter" condition. The dose distributions calculated with FBCT and CBCT images compared in a gamma index evaluation of 1%/3 mm criteria and in the dose difference. With the electron density correction acquired in the same scatter condition, the CBCT based dose calculations tended to be the most accurate. In 5 prostate cases in which the mean equivalent diameter was 27.2 cm, the averaged gamma pass rate was 98% and the dose difference confirmed to be less than 2% (average 0.2%, ranged from -1.3% to 1.6%) with the electron density correction of the "Full-scatter" condition. The accuracy of CBCT based dose calculation could be confirmed that closely related to the CT number uniformity and to the similarity of the scatter conditions for the electron density correction curve and CBCT image. In pelvic cases, the most accurate dose calculation was achievable in the application of the electron density curves of the "Full-scatter" condition

KeywordsCBCT, Dose calculation, Electron density phantom, Pelvic radiotherapy

Korean Society of Medical Physics

Vol.35 No.2
June 2024

pISSN 2508-4445
eISSN 2508-4453
Formerly ISSN 1226-5829

Frequency: Quarterly

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