探討準直儀互換效應對Varian 21EX醫用直線加速器輸出劑量之影響

空氣中的輸出比S(下標 c)是計算輸出監控單位(MU)中的一個重要參數。計算監控單位可利用非射源產生之輻射的原理，導出公式(方程式略)並接著加以計算，公式中K值為幾何比重因子，Varian 21EX直線加速器為1.49。測量正方形4×4平方公分-40×40平方公分照野所測得之結果，光子能量為6MV的S(下標 c)是由0.945至1.054；10MV則由0.933至1.055。長方形照野方面，光子能量為6MV的S(下標 c)在計算值與測量值之間的誤差範圍在－0.82%-0.69%，而10MV則在－0.98%-0.96%。比較準直儀互換效應(C(下標 e))的測量值，最狹長形之照野（Y軸＝4cm, X軸＝40cm）與（Y軸＝40cm, X軸＝4cm）偏差最為明顯為3.9%。K值越接近1時，C(下標 e)會越不明顯。C(下標 e)本身主要是照射時準直儀回散射影響監控式游離腔所造成，並與準直儀、監控式游離腔的設計及K值有關，不同廠牌型號直線加速器的機頭設計不同而C(下標 e)會有差異，所以期待將來所設計的醫用直線加速器能改善或解決C(下標 e)的問題。

關鍵字

空氣中的輸出比；非射源產生之輻射；準直儀互換效應

並列摘要

The output ratio in air or the collimator scatter factor (S(subscript c)) is an important parameter for the calculation of monitor unit. The principle of extrafocal radiation could be used for the calculation of monitor unit. An equation, (The equation is abbreviated), is derived to calculate the equivalent field size of a rectangular field, where k is the geometric weighting factor. The k for the linac Varian 21EX is 1.49. The Sc measured ranges from 0.95to 1.054, and 0.933 to 1.055 for 6 MV and 10 MV photon respectively, for square fields ranging from 4 × 4 cm^2-40 × 40 cm^2. For rectangular field sizes, the deviation found between the calculated and measured Sc was -0.82%-0.69% and -0.98%-0.96%, for 6 MV and 10 MV respectively. The value k plays a significant role for collimator exchange effect (C(subscript e)). Comparison of C(subscript e) measurements, a maximum deviations 3.9% was obtained for most elongated field sizes with (Y=4 cm, X=40 cm) and (Y=40 cm, X=4 cm). The correlation is obscure when K approximates to 1. Variations in C(subscript e) may be caused by the backscatter of collimators to the monitor ion chamber, design of ion chamber, collimator and K values, which are accelerator dependent. Further effort of the linear accelerator manufacturers is needed to improve and reduce the C(subscript e).

並列關鍵字

output ratio in air ； extrafocal radiation ； collimator exchange effect

國際替代計量

探討準直儀互換效應對Varian 21EX醫用直線加速器輸出劑量之影響

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