MCNPモンテカルロコードを用いた^<60>Co照射装置のスペクトルと出力計算  [in Japanese] ^<60>Co ENERGY SPECTRUM AND OUTPUT CALCULATION BY USING MCNP MONTE CARLO CODE  [in Japanese]

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近年,医療では,外部照射装置として直線加速器が主に使われているが,コバルト装置はガンマ線出力が安定しているため,線量計の校正や基準線量として利用されている.コバルト装置の基本特性を汎用モンテカルロコードMCNPで解析した.エネルギースペクトル,出力係数(OPF)と深部線量百分率(PDD)をモンテカルロ計算し,電離箱線量計との比較を行った.比較の結果は4.8%以内の一致となり,モンテカルロ計算が有用であった.また広島大学原爆放射線医科学研究所ではコバルト照射装置が生物学的試料(細胞や動物)を照射する目的で使われてきた.その際,低線量,低線量率の照射は鉛板で減衰させて行われてきたが,鉛板による散乱線の増加で線質の変化が危惧されてきた.今回,鉛板の厚さが 1~5 cmの範囲で透過したスペクトルのモンテカルロ計算を行った.この結果, 鉛板の厚さによる大きな変化がないことが分かった.モンテカルロ計算は実測が困難な場合や実測の検証に有効であり,治療計画装置のアルゴリズムにも組み込まれてきている.今後,このモンテカルロ計算を使った手法は,他種の治療装置の計算,品質保証(QA),品質管理(QC)に広く利用されることが予想される.

In a field of the recent medical care, linear accelerators are mainly used as an external beam radiotherapy device. On the other hand, cobalt-60 irradiation apparatuses are used for dose calibration of absolute dosimeters such as ionization chambers because of its good stability and clearly known energy spectrum comparing with those of linear accelerators. At the Research Institute for Radiation Biology and Medicine, Hiroshima University, a cobalt-60 apparatus (111 TBq) has been installed for the use of radiation biology study. We calculated basic output characteristics of this cobalt-60 irradiation apparatus using the Monte Carlo MCNP code. The energy spectrum of the cobalt-60 was used as an input of the MCNP code and, output factor (OPF) and percentage depth dose (PDD) were obtained as basic characteristics. We compared calculation with the measured results using a tertiary standard ionization chamber. The agreement between the calculation and measurements were less than 4.8%. Also in the case of this cobalt-60 apparatus low dose-rate experiment sometimes performed to irradiate cells, mice and etc. To reduce the dose rate, Pb plates were used. In this paper we calculated such spectra after absorption of the Pb plates to obtain the aspects of the scattering gamma rays. Actual energy spectra in the phantom etc. are not easy to measure. However sometimes it is necessary to know energy spectra to understand dose distribution or such characteristics precisely. In future this method will be widely used for the output calculation and for quality assurance (QA) and quality control (QC) of radiotherapy treatment planning system.


  • J Jpn Soc Ther Radiol Oncol  

    J Jpn Soc Ther Radiol Oncol 18(4), 209-215, 2006-12-25 

    Japanese Society for Therapeutic Radiology and Oncology

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