現今放射治療使用的治療床已改成質地更堅固且載重負荷更佳的碳纖維材質；本實驗的目的就是要探討當直 線加速器的治療射束經過碳纖床時的衰滅性質及其修正因子的探討。本實驗使用的是Varain ix的直線加速器及其碳纖床，及一套自行設計的半徑為11公分的球形假體；以農夫型游離腔量測，當加速器的機頭角度每隔5°取一個游離腔讀值，逐一量測當射束由下而穿透碳纖治療床的碳纖材質時輻射的衰減量，及如何還原當加速器的機頭在任意角度下，藉由此篇所提供的角度對輻射的劑量修正此衰減值的探討。

The purpose of this study was to evaluate the effect and to propose a method for correction of megavoltage photon beam attenuation by treatment couch tops. The study was performed on Varian Exact carbon fiber couch top with a ball phantom by inserting Farmer chamber for measurement. Four sites along couch midline was measured to check the photon attenuation at different angle with fully gantry rotation. Attenuation was observed maximum to 4.9% normalized at isocenter at angle in between 0o-60o below horizontal. The equivalent thickness of carbon fiber couch top at a speciffic gantry angle was maximum 9.2mm derived from phantom measurement. A simple method using the proposed equation and a correction factor should be useful to correct the isocenter dose and should be considered an RTP. This method should be used to correct the isocenter dose in actual treatment by inserting the equation to the dose calculation equation in RTPs.