Investigation of the influence of the conversion method to equivalent square field on the depth direction of phantom scatter factor : Measurement in Synergy linear accelerator

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  • 等価正方形照射野への変換法の違いがファントム散乱係数の深さ方向に与える影響の検討 : 線形加速器Synergyにおける計測
  • トウカ セイホウケイ ショウシャヤ エ ノ ヘンカンホウ ノ チガイ ガ ファントム サンラン ケイスウ ノ フカサ ホウコウ ニ アタエル エイキョウ ノ ケントウ : センケイ カソクキ Synergy ニ オケル ケイソク

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Abstract

X線治療の計算によるMU独立検証で等価正方形照射野（等価正方形辺）は，検証の精度に影響するため重要である．等価正方形辺は深さやエネルギーに依存しないと仮定されている．しかし，先行研究で線形加速器Clinac-iX（Varian社製）の矩形照射野で求めたファントム散乱係数S_pの測定値と，同じ矩形照射野を等価正方形辺に変換して求めたS_pの推定値の差は深さによって変わる深さ依存があり，その仮定は有効ではなかった．また，Day法やA/P法に比べARC法の等価正方形辺はその深さ依存が小さいことも報告された．本研究は，Clinac-iXとガントリヘッドの構造が異なる線形加速器Synergy（Elekta社製）で，同じ方法によりS_pの測定値と推定値の差を調べた．その結果，Synergyでも深さ依存が見られ，ARC法がDay法やA/P法に比べて小さく，S_pの推定精度が良いことが示された．

In X-ray therapy, equivalent square field (side of equivalent square field) is important because it influences the accuracy of monitor unit (MU) independent verification by calculation. It is assumed that equivalent square field does not depend on depth and X-ray energy. However, in Clinac-iX (manufactured by Varian Co.) linear accelerator of previous research, the difference between measured value of S_p obtained by measuring rectangular field and estimated value of S_p obtained from equivalent square field of same rectangular field　indicated depth dependency that varied with depth, and it was suggested that the assumption was not valid. And it was reported that the depth dependency in side of equivalent square of ARC method was smaller than Day method and A/P method. In this study, using Synergy (manufactured by Elekta Co.) that was linear accelerator with different structure in the gantry head of Clinac-iX investigated the difference between measured value and estimated value of S_p by the same method of previous research. As result, the depth dependency was found also in Synergy, and it was shown that side of equivalent square of ARC method was smaller than Day method and A/P method, and the estimation accuracy of S_p was better.

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