Progress in Medical Physics (한국의학물리학회지:의학물리)

Korean Society of Medical Physics (한국의학물리학회)
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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

  • Yoon, KyoungJun (Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of Medicine) ;
  • Kwak, Jungwon (Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of Medicine) ;
  • Cho, Byungchul (Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of Medicine) ;
  • Kim, YoungSeok (Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of Medicine) ;
  • Lee, SangWook (Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of Medicine) ;
  • Ahn, SeungDo (Department of Radiation Oncology, Asan Medical Center, University of Ulsan College of Medicine) ;
  • Nam, SangHee (Department of Biomedical Engineering, University of Biomedical Science and Engineering)
  • 윤경준 (울산대학교 의과대학 서울아산병원 방사선종양학과) ;
  • 곽정원 (울산대학교 의과대학 서울아산병원 방사선종양학과) ;
  • 조병철 (울산대학교 의과대학 서울아산병원 방사선종양학과) ;
  • 김영석 (울산대학교 의과대학 서울아산병원 방사선종양학과) ;
  • 이상욱 (울산대학교 의과대학 서울아산병원 방사선종양학과) ;
  • 안승도 (울산대학교 의과대학 서울아산병원 방사선종양학과) ;
  • 남상희 (인제대학교 의생명공학대학 의용공학과)
  • Received : 2014.02.24
  • Accepted : 2014.03.16
  • Published : 2014.03.31
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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.

ConeBeam Computed Tomography (CBCT) 영상을 기반으로 한 선량계산에서는 Fanbeam Computed Tomography (FBCT)와 비교하여 산란에 의한 영향이 크고 그 양상이 다양하게 나타나 오차의 주요한 요인으로 작용하는 것으로 알려져 있다. 본 논문에서는 골반 방사선 치료에서 산란이 CBCT 기반으로 한 선량계산에 미치는 영향을 평가하여 오차를 최소화 할 수 있는 조건에 대하여 연구하였다. 다양한 산란조건에서의 CBCT 영상 취득을 위하여 전자밀도 교정용 팬텀에 크기가 각기 다른 산란물질을 추가하여 "산란부족", "산란과다", 그리고 "산란충분"의 3가지 조건을 정하였다. 산란조건에서 취득된 CBCT 영상에서 팬텀 중심부와 주변부의 위치에 따른 CT number값의 차이와 분포를 분석하여 균질도를 평가하였으며 FBCT 영상 기반의 선량 분포를 기준으로 하여 다양한 산란조건에서의 전자밀도 교정관계를 적용하였을 때 팬텀 및 전립선암 환자 5명의 CBCT 영상에서 계산된 선량분포의 감마합격률 및 상대적 오차를 구하였다. 팬텀 CBCT 영상에 대한 CT number들의 히스토그램에서의 분포에서 물 등가 물질에 해당하는 피크의 폭(FWHM)은 산란부족(685 HU)이나 산란과다(264 HU)보다 산란충분(146 HU)의 조건에서 가장 작게 나타나 균질도가 제일 좋은 것으로 평가되었고 팬텀의 중심부와 주변부에서 동일 성분에 대한 CT number의 차이 역시 같은 결과를 나타내었다. 또한 팬텀의 CBCT 영상을 취득할 때와 동일한 산란조건에서의 교정조건을 적용한 경우 선량계산이 가장 정확하였으며 산란충분의 교정곡선 조건을 적용하였을 때 5명의 전립선암환자(평균 등가지름 27.2 cm)의 CBCT 영상 기반의 선량분포는 FBCT의 경우와 대비하여 1%/3 mm의 감마지표에서 감마합격률 98% 이상을 나타내었다. 이때 FBCT 선량에 대한 CBCT 선량오차는 처방선량 대비 2% 이하(평균 0.2%, -1.3%~1.6%)로 평가되었다. CBCT 골반 촬영을 할 때 일반적인 성인 골반의 원통 등가지름(ECD, Equivalent Cylindrical Diameter)의 산란조건에서 동일 성분에 대한 HU 값이 가장 균질하게 나타나 골반 촬영모드가 최적화되었음을 확인하였으며 일반적인 골반부위와 ECD가 유사한 산란조건, 즉 산란충분조건에서 취득된 전자밀도 교정관계를 적용하여 골반 CBCT 기반에서 선량을 계산하였을 때 최적의 선량 정확성을 확보할 수 있었다.

Keywords

References

  1. Xing L, et al: Overview of image-guided radiation therapy. Med Dosim 31(2):91-112 (2006) https://doi.org/10.1016/j.meddos.2005.12.004
  2. Hector CL, Webb S, Evans PM, et al: The dosimetric consequences of inter-fractional patient movement on conventional and intensity-modulated breast radiotherapy treatments. Radiother Oncol 54(1):57-64 (2000) https://doi.org/10.1016/S0167-8140(99)00167-X
  3. Coolens C, et al: The susceptibility of IMRT dose distributions to intrafraction organ motion: an investigation into smoothing filters derived from four dimensional computed tomography data. Med Phys 33(8):2809-2818 (2006) https://doi.org/10.1118/1.2219329
  4. Hugo GD, Agazaryan N, Solberg TD, et al: The effects of tumor motion on planning and delivery of respiratory-gated IMRT. Med Phys 30(6):1052-1066 (2003) https://doi.org/10.1118/1.1574611
  5. Yoo S, Yin FF, et al: Dosimetric feasibility of cone-beam CT-based treatment planning compared to CT-based treatment planning. Int J Radiat Oncol Biol Phys 66(5):1553-61 (2006) https://doi.org/10.1016/j.ijrobp.2006.08.031
  6. Ding GX, et al: A study on adaptive IMRT treatment planning using kV cone-beam CT. Radiother Oncol 85(1):116-25 (2007) https://doi.org/10.1016/j.radonc.2007.06.015
  7. Siewerdsen JH, Jaffray DA, et al: Cone-beam computed tomography with a flat-panel imager: magnitude and effects of x-ray scatter. Medical physics 28(2):220-231 (2001) https://doi.org/10.1118/1.1339879
  8. Lee L, Le QT, Xing L, et al: Retrospective IMRT dose reconstruction based on cone-beam CT and MLC log-file. Int J Radiat Oncol Biol Phys 70(2):634-344 (2008) https://doi.org/10.1016/j.ijrobp.2007.09.054
  9. Guan H, Dong H, et al: Dose calculation accuracy using cone-beam CT (CBCT) for pelvic adaptive radiotherapy. Phys Med Biol 54(20):6239-6250 (2009) https://doi.org/10.1088/0031-9155/54/20/013
  10. Yang Y, et al: Evaluation of on-board kV cone beam CT (CBCT)-based dose calculation. Phys Med Biol 52(3):685-705 (2007) https://doi.org/10.1088/0031-9155/52/3/011
  11. Rong Y, et al: Dose calculation on kV cone beam CT images: an investigation of the Hu-density conversion stability and dose accuracy using the site-specific calibration. Med Dosim 35(3): 195-207 (2010) https://doi.org/10.1016/j.meddos.2009.06.001
  12. Masahiro Endoa, et al: Effect of scattered radiation on image noise in cone beam CT. Med Phys 28:469 (2001) https://doi.org/10.1118/1.1357457
  13. Calvo Ortega JF, et al: A dosimetric evaluation of the Eclipse AAA algorithm and Millennium 120 MLC for cranial intensity- modulated radiosurgery. Med Dosim (2013)
  14. Lu W, et al: Deformable registration of the planning image (kVCT) and the daily images (MVCT) for adaptive radiation therapy. Phys Med Biol 51(17):4357-4374 (2006) https://doi.org/10.1088/0031-9155/51/17/015
  15. Matsinos E, et al: Current status of the CBCT project at Varian Medical Systems Proc. SPIE 5745:340-351 (2005)