以直線加速器所產生之光子與電子束進行放射治療是現今應用於臨床最廣泛之放射治療技術。而爲了提高皮膚免除效應，使治療深層腫瘤時可有效降低皮膚及表淺正常組織之劑量，直線加速器之光子治療能量有逐漸提高之趨勢。一但光子能量高過10MV以上時，則提高了光核反應(photonuclear reaction)的機率，進而產生出中子與質子等輻射：因此，就高能量之直線加速器而言，其所產生之輻射不僅包括低線性能量轉移(linear energy transfer, LET)之光子與電子，還含有高LEQT的中了與質子等輻射，這些不同輻射所造成之生物劑量特性，已成爲使用10MV以上高能量直線加速器進行放射治療所必須面對的重要課題。依據微劑量學的理論、考慮直線加速器產生之高能量光子以及中子與質子等高LET輻射粒子之特性，本研究以組織等效比例計數器模擬2μm大小的細胞進行直線加速器之臨床光子放射治療時之線性能量分佈，以評估臨床高能光子放射治療之作用與效應。本研究藉由利用組織等效比例計數器模擬度量高能直線加速器產生之不同輻射與靶物質所產生的作用與效應，由y與yf(y)分佈之結果，可看出不同LET幅射在靶細胞中發生作用的頻率，而y與yd(y)分佈之結果則可了解6MV X-ray之劑量貢獻較爲單純，僅由光子所造成，15MV X-ray因會發生光核作用而產生中子，因此有少部分之劑量貢獻是來自於光子以外的中子污染。

Photons and electrons produced by the linear accelerator (Linac) are the most popular radiation particles in clinical application of radiotherapy To arise the skin sparing effect, decrease the skin doses and shallow tissue doses effectively as treating deeper tumors, the photon beams of the Linac are intentioned to have higher energy. As the photon energy is beyond 10 MeV, probabilities of photonuclear reactions are increased and secondary particles such as neutrons and protons are then following produced. Thereby, the particles produced by a high energy Linac will include high linear energy transfer (LET) particles, such as neutrons and protons, and low LET particles, such as photons and electrons. Characteristics of biological doses induced by different radiation particles that produced by a Linac became more and more important in radiotherapy. Based on microdosimetry theory and characteristics of high energy photons and high LET radiations such as neutrons and protons, this work applied a tissue equivalent proportional counter (TEPC) to simulate and assess the interactions and effects as a 2 m cell was irradiated by the high energy photons produced by the Linac. In the results, interactions and effects of different radiations were assessed by lineal energy distributions, yf(y) and yd(y). By yf(y) distribution, important in frequency of different LET radiation is presented. Beside, by yd(y) distribution, dose contribution of different LET radiation show that 15 MV photon beam induce exactly a few high LET radiations.