日本放射線技術学会雑誌
Online ISSN : 1881-4883
Print ISSN : 0369-4305
ISSN-L : 0369-4305
原著
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
早川 岳英山田 巧坂井 裕則笹本 龍太
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2019 年 75 巻 6 号 p. 525-535

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In X-ray therapy, equivalent square field (side of equivalent square field) is important because it influences the accuracy of independent verification of monitor unit (MU) by calculation. To calculate the side of equivalent square field for rectangular fields, we often use a table of domestic standard measurement method (Day’s method), or A/P method calculated by area-perimeter ratio. The sides of equivalent square fields of these methods are assumed to be unchanged by depth and energy, but there are reports that it is not valid. Therefore, the depth dependency of side of equivalent square fields of Day’s method, A/P method, and area ratio correction (ARC) method was compared by measuring phantom scatter factors (Sp). From the analysis of Sp measured at different depths, the estimated value of Sp on the equivalent square side of the Day’s method and A/P method had a depth dependency that the difference from the measured value was large when the measurement depth was deep. The estimated value of Sp on the equivalent square side of the ARC method had a small difference from the measured value even when the measurement depth was deep, and the depth dependency was small compared with the Day’s method and the A/P method. Side of equivalent square field of ARC method had a smaller difference of depth dependency than in the case of Day’s method and A/P method. Therefore, in the independent verification of MU for rectangular field, using the equivalent square side of the ARC method is better.

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