前言:在臨床的放射治療中使用電子射束時，會依據不同情形而使用不同的電子錐，而在使用電子錐之後，以及電子錐和假體表面之間有無空氣間隙的存在，電子射束的特性都會有所改變，故本篇針對Siemens Linac MD－2直線加速器使用不同的電子錐時，所得到的電子射束特性變化倣一研究探討。10*10cm□電子錐以及直徑5cm的圓形照野電子錐等三種不同的電子錐裝置在12MeV、80MU的條件下，分別於電子錐和假體表面之間沒有空氣間隙(air gap=0cm)，以及空氣間隙等於10cm的情形下進行照射，並針對其所得結果相互做一比較。化的劑量梯度(G□)和d□亦較大，而且均勻度較好。如果是使用裝有直徑5cm圓形照野遮擋鉛塊的10*10cm□電子錐時，所造成的均勻度比較差，均一化的劑量梯度(G□)和d□亦較小，但是背景X射線和表面劑量會較小。當電子錐和假體表面之間的空氣間隙增加時(airgap=0cm→air gap=10cm)，半影區的寬度變大，背景X射線和散射也增加了。並且發現空氣間隙的存在與否，對使用10*10cm□正方形照野的電子錐所造成的影響較大。腫瘤;如果是使用裝有直徑5cm圓形照野遮擋鉛塊的10*10cm□電子錐時，用來治療小一點的腫瘤比較適當。而使用裝有直徑5cm圓形照野遮擋鉛塊的10*10cm□電子錐所得之電子射束的均勻度和有效治療面積，比使用直徑5cm的圓形照野電子錐來得好。

　　　　　Introduction: When electron beams are used for radiotherapy, the treatmentportals are often shaped to fit the treatment area. In some clinical application,a larger nominal gap is necessary. For these different applicator effects, we mustdetermine empirically these effects for the applicator system employed.with 5 cm diameter circular cutout, and a 5 cm diameter open cone for 12 MeV, 80 MUelectrons will be studied. Change the air gap between the collimator and the phantomsurface from 0 cm to 10 cm. We measure parameters and compare these differences.background, larger G□ and flatness is better. If the applicator is a 10 * 10 cm□ cone with 5 cm diameter circular cutout, the X-ray background is smaller. As theair gap between the collimator and the phantom surface increases the penumbraincreases, X-ray background and scatters increase, too. We also find that the 10 *10 cm□ open cone for the effect of the air gap is larger.larger tumors. To treat smaller tumors, a 10 * 10 cm□ cone with 5 cm diameter circularcutout will be better. The flatness and effective treatment size of electron beamswith a 10 * 10 cm□ cone with 5 cm diameter circular cutout are better than with a5 cm diameter open cone.