本文應用磁場對帶電粒子作用的原理，藉由磁場羅倫茲力作用偏移二次帶電粒子的方向，減少因爲直線加速器機頭部分組件以及鉛合金擋塊托盤等所引起病人劑量，進而改變光子射束百分深度劑量曲線，作爲臨床應用的參考。使用西門子MD-2直線加速器所產生的6MV及10MV光子射束，於機頭附加上一組磁鐵中心強度爲2400gauss（即0.24tesla）的稀土類永久磁鐵，以平行板游離腔針對架設鉛合金擋塊扎盤之有無進行輻射劑量量測，作外加磁場對百分深度劑量影響評估。結果顯示，治療機頭加設磁場前後，各組實驗的百分深度劑量曲線在劑量增建區都有明顯的變化（其電子污染變化範圍最多可達8.771%），但是在增建區之後劑量相同。當治療機頭未架上鉛合金擋塊托盤(tray)時？能量較高的光子，其增建區劑量降低的幅度也較大。經過與未加托盤的劑量比較後可得知：加托盤後增建區劑量會比未加托盤時高，但是若托盤照野中加設磁場，則增建區劑量降幅比未加托盤的照野增建區劑量降幅更多。在機頭附加上磁場時，只會減少增建區的劑量，並不會改變深部治療劑量。因此即使其減少幅度各區不一，仍不影響臨床治療劑量的計算。因此治療射束中附加上磁場以改變治療射束的百分深度劑量曲線，並進而減少淺表劑量可算是一個好用且方便的方式。

The purpose of this study is to reduce the dose in buildup region, which is caused by secondary electrons induced by part of the head of linear accelerator, shielding blocks and trays. The basic principle is that the direction of the charged particles can be affected by Lorentz force of the magnetic field. The trajectory of secondary electrons can be manipulated resulting in reduced shallow dose of patients. The measurement is performed using a Siemens MD-2 linear accelerator equipped with 6 MV and 10MV photons. A pair of permanent magnets with 2,400gauss (0.24tesla) is mounted on the head of linear accelerator. We use parallel plate ionization chamber to evaluate the influence of magnetic field upon the surface. In the case of magnetic field, the degree of electron contamination is from 0.998% to 8.771%. In the case without block tray, the degree of electron contamination increases with higher photon energy and field size. The percentage depth dose (PDD) varies obviously in buildup region, and keeps constant beyond that region In the case with block tray, the dose in buildup region is higher than that without tray, and the degree of electron contamination of buildup region dose increases with magnetic field. When we add magnet on the head of the linear accelerator, the percentage depth dose varies markedly only m buildup region, and keeps constant beyond that region. It is not required to change the calculation program. It is concluded that the magnetic device is convenient and applicable clinically.