現今放射治療的方式常常會讓射束斜向由下往上照射，而此時射束可能會經過一塊厚度不同的壓克力板，此塊壓克力板有可能是頭部枕墊的支撐板或是病人頭部懸空時的支撐板；而此時射束中心軸斜向所經過的幾何距離是否可代表就是經過相同厚度的組織？因此射束斜向經過壓克力板的劑量修正因子即爲本篇所要探討的主題。我們以Farmer游離腔以及Keithley的微電量儀在固態假體中對厚度爲0.5公分、1.0公分和1.5公分的壓克力板，以射束手口壓克力入射的角度分別爲00、150、300、450及600分別測量不同的游離腔的讀值而測量的條件爲SAD爲100公分，照野開10公分x10公分及20公分x20公分，以沒加壓克力的結果和有加壓克力的讀值比，用來做爲修正因射束斜照經過較長的途徑所引起的劑量之阻擋。結果如下，照野爲10公分x10公分酌情形，MU之修正量最大會大到8.9%，而20公分x20公分最大會大到7.8%；實驗結果顯示，照野愈小入射角愈大則劑量的差異會愈大；結論如下，利用簡單的方法求得射束斜向穿透壓克力板時有效厚度的修正量，以期能在治療病人時能夠給予更精確的將劑量。

Conformal therapy is not contented with conventional coplanner beams delivery technique because of the confinement in beams direction. Non-coplanner beams dose delivery technique usurp conventional coplanner technique to become the essential treatment technique in conformal therapy. In non-coplanner treatment patient usually has to lie on a acrylic board to support his suspending body on treatment couch. In this experiment, we try to measure the attenuation effect and determine the dose correction factor of the acrylic board while radiation beams penetrate obliquely through the acrylic supporter. We use acrylic board with different thickness from 0.5cm to 1.5 cm, Farmer 0.6c.c, ion chamber and keithley electrometer in solid phantom. The photon beam entry angles were 0�, 15�, 30�, 45� and 60� respectively. The field sizes were 10cm(superscript *) 10cm and 20cm(superscript *) 20cm with SAD 100cm. The ratio of the reading of the open beams and acrylic board-attenuated beams was obtained. The dose correction factor of the acrylic board was obtained from the electrometer reading If we adopt the geometry thickness to calculate the monitor unit, there will be 8.9% to 7.8% under dose with field size ranged from 10cm x 10cm to 20cm x 20cm, When radiation beams penetrate obliquely through acrylic board the dose correction should be taken into consideration in the calculation of treatment monitor unit.