醫用直線加速器在臨床使用時會運用到加速器的兩種輻射源，一種是高能光子射束，另一種電子射束；當實驗使用直線加速器所產生的電子射束治療病患時，在加速管內的電子被加速之後並沒有讓電子撞擊到金屬靶，而是讓被加速的電子束經過一個散射薄片(scatter foil)後使電子束平均的散開來以便做爲臨床治療使用；由於電子到達病人前會經過機頭金屬物及會和電子在行進過程中所碰到的空氣做碰撞；所以在假體內將電子輻射劑量的分佈情形以物理特性反推時，會發現真正被加速的電子和散射薄片碰撞的點並不是所謂的電子和散射薄片的碰撞點之射源點(focal spot)，而是有一虛擬射源位置點的存在；而本篇主要探討的主題即是電子射線劑量計算及分佈受到虛擬射源位置點的影響，藉由實驗探討電子射線的照野及能量的變化對虛擬射源位置點的影響。

Electron beam do not emanate from a physical source in the accelerator like x-ray beam, Instead the electron beam is spread by the scattering foils into a broad beam that appears to diverge from a point, usually referred to as the virtual source. In this study, how electron dose calculation affected by virtual source position in planning system has been discussed. The position of the virtual source can be found by back projection of the 50% width of the beam profile (FWHM) obtained at different source to chamber distance. Project the best fit line to the width of zero (FWHM=0) from different reduced blocked field size in the same cone and same energy will gather different virtual source position results. With the same cone size but different reduce fabricated fields resulted in different virtual source position. Subtraction the source to chamber distance from the calibration setup SSD to obtain in the same electron energy but different cone aperture would be approximately has the results of varying from 93.5cm to a wide range of 40cm. User must calculate the virtual SSD for each electron energy and each cone aperture of different reducing fabricated fields, in other word, there would be several sets of virtual source position to be implemented instead of only one virtual source position value when commission the treatment planning system.