電子射線PENCIL BEAM原理劑量計算之評估

Dosimetric Evaluation of a Pencil Beam Algorithm

葉健一(Jam-I Yeh)

《中華放射線醫學雜誌》 18卷3期 (1993/09) Pp. 253-261

摘要

本研究目的在探討一般臨床電子射線pencil beam計算原理，除將此原理一些基本假設與物理意義作詳細說明外，並與臨床實驗結果作比較以評估此原理之準確性。1981年Hogstrom等人將pencil beam原理成功運用於電腦計劃系統裡，利用一部份測量數據簡化pencil beam原理複雜的數學公式與理論上不足之處，一般而言pencil beam原理對於表面平坦的均勻物質劑量預測非常準確（誤差＜5mm），但在表面彎曲或不均物質(inhomogeneities)情況下就出現較多的誤差，此現象的產生完全在於原理中兩項基本假設的限制：1)電子射線在物質裡完全以小角度散射而不考慮可能的大角度散射現象。2)每一條pencil beam在物質內造成的劑量只與中軸(central axis)所經過的物質有關，並沒考慮離軸(off-axis)上的非均勻物質，因此在實驗比較結果中可發現在不同物質交接處pencil beam原理的預測上有相當的誤差出現。

關鍵字

電子射線，劑量學 ； 輻射科學

並列摘要

The purpose of this research is to evaluate the algorithm of pencil beam in electron dose calculation. The basic assumptions and physics meanings of pencil beam algorithm will be discussed and the experimental results will also be presented. In 1981, pencil beam algorithm was first implemented in electron dose calculation using computer treatment planning system by Hogstrom et al. The Hogstrom algorithm has constituted a simple and tremendous advance in electron dose calculation. In general, the estimated dose in homogeneous medium using pencil beam algorithm can achieve an accuracy of 5 mm. However, it is not yet possible to calculate the dose distribution precisely, particularly in the presence of tissue inhomogeneities and body curvature. There are two main problems in this algorithm: 1) it describes the behavior of the electron beam as a small angle scattering and neglects the effect of the large angle scattering of primary electron and 2) it calculates dose by summing pencil beam dose distributions, which are calculated as if the inhomogeneity structures underlying the central ray of a pencil beam are infinite in their lateral extent. Therefore, we found the lack of accuracy in the vicinity of inhomogeneities.

並列關鍵字

Electron beam ； dosimetry ； radiation science