Yamagata University / 이성현*
Abstract
The errors on the stopping power ratio (SPR) of mouthpiece samples from ERKODENT were evaluated. Erkoflex and Erkoloc-pro from ERKODENT and samples that combined Erkoflex and Erkoloc-pro were computed tomography (CT)-scanned using head and neck (HN) protocol at the East Japan Heavy Ion Center (EJHIC), and the values were averaged to obtain the CT number. The integral depth dose of the Bragg curve with and without these samples was measured for 292.1, 180.9, and 118.8 MeV/u of the carbon-ion pencil beam using an ionization chamber with concentric electrodes at the horizontal port of the EJHIC. The average value of the water equivalent length (WEL) of each sample was obtained from the difference between the range of the Bragg curve and the thickness of the sample. To calculate the difference between the theoretical and measured values, the theoretical CT number and SPR value of the sample were calculated using the stoichiometric calibration method. Compared with the Hounsfield unit (HU)-SPR calibration curve used at the EJHIC, the SPR error on each measured and theoretical value was calculated. The WEL value of the mouthpiece sample had an error of approximately 3.5% in the HU-SPR calibration curve. From this error, it was evaluated that for a mouthpiece with a thickness of 10 mm, a beam range error of approximately 0.4 mm can occur, and for a mouthpiece with a thickness of 30 mm, a beam range error of approximately 1 mm can occur. For a beam passing through the mouthpiece in HN treatment, it would be practical to consider a mouthpiece margin of 1 mm to avoid beam range errors if ions pass through the mouthpiece.
Affiliations
Sung Hyun Lee 1, Takayuki Kanai 1 2, Hikaru Souda 1, Yuya Miyasaka 1, Hongbo Chai 1, Takuya Ono 1, Yoshifumi Yamazawa 1 3, Koji Suzuki 3, Azusa Sato 4, Masashi Katsumata 4, Takeo Iwai 1
1Department of Heavy Particle Medical Science, Graduate School of Medical Science, Yamagata University, Iidanishi, Yamagata, Japan.
2Department of Radiation Oncology, Tokyo Women's Medical University, Shinjuku, Tokyo, Japan.
3Department of Radiology, Yamagata University Hospital, Iidanishi, Yamagata, Japan.
4Accelerator Engineering Corporation, Inage, Chiba, Japan.
편집위원
본 논문은 두경부암의 중입자치료시 환부 고정에 사용되는 마우스피스의 저지능비(stopping power ratio: SPR)에 대해 연구하였으며, 기존의 방식을 적용한 경우에 선량계산에서 비정 오차(range error)가 약 1 mm까지 발생할 수 있음을 제시하였다.
SPR은 하전입자가 매질을 통과할 때, 매질에서의 에너지손실과 물(water)에서의 에너지손실의 비를 의미하는데, 치료계획시스템에서는 환자의 CT 영상으로부터 HU-SPR 관계를 이용하여 SPR을 구하여 인체내 선량에 적용한다. 이 때 HU가 오로지 감쇠계수(attenuation coefficient)만으로 결정되는 인자이기 때문에 물질의 종류를 구분하기 어렵고, 이로 인해 특정 물질에서 SPR의 오차를 유발할 수 있고 이는 곧 선량계산의 오차로 연결된다.
본 논문에서는 실험적 연구를 통해 마우스피스의 두께가 10 mm인 경우에 0.4 mm, 그리고 30 mm 인 경우에 약 1 mm까지의 비정 오차가 발생함을 발견하였고 치료계획시 마진(margin) 설정에 이를 반영해야 한다고 권고하였다. 본 논문은 탄소빔 중입자치료 분야에서 작성되었지만, 체내에 보조기구를 삽입하는 일반적인 방사선치료 분야에도 방사선치료의 정확성 향상 연구에 유익한 참고문헌이 될 수 있다.
2023-07-05 14:27:34