Varian直線加速器電子射束有效射源至表面距離之測量研究

A Study Measuring the Effective SSD of Varian Linear Accelecators

黃英明(Ing-Ming Hwang) ； 梁永昌(Henry W. C. Leung)

《放射治療與腫瘤學》 3卷2期 (1996/06) Pp. 113-120

https://doi.org/10.6316/TRO/199603(2)113

摘要

目的：本篇測量三部不同型號之Varian雙能量高能直線加速器電子射束之虛擬射源位置，求出其有效射源至表面距離，提供臨床上以延長SSD方式治療時計算劑量及監測單位之用，同時觀察並比較此三部機組因設計及配備不同造成有效射源至表面距離不同之處。
材料與方法：將Scanditronix RFA-300水假體組安裝於加速器電子限光筒下方，再將N.E. 0.03c.c.平板型游離腔固定於水箱偵測器支架上，將游離腔表面調整至射東中心軸最高劑量點處，以lonex Dosemaster 2590電位計，分別於光距離指示器為100、110及120cm處，就各能量，各電子限光筒測量其讀值，最後將測量值依反平方定律求得其有效射源至表面之距離。
結果：射束能量愈低，限光筒愈小，ESSD愈短；射束能量相同，限光筒愈大，ESSD愈大且愈接近100cm；相同的電子限光筒，能量愈高，ESSD並不成比例關係變化；MLC 型限光筒形成的ESSD變化較小；使用Kapton型監測游離腔之機型形成的ESSD比由Mica型游離腔所形成的ESSD較為接近100cm。
結論：本量測之結果可應用於臨床上以延長SSD方式治療病患時，劑量計算之修正參數之一；又由本量測可知：就Varian直線加速器而言，機型相同，電子射東能量相同，限光筒不同，形成的ESSD不同不機型不同，能量相同，限光筒相同，形成的ESSD不同；機型不同，能量相同，限光筒不同，所形成的ESSD亦不同：電子射束之ESSD應隨機量測，並無一定的變化結果及完全相同的數值。

關鍵字

放射治療 ； 電子射束 ； 虛擬射源位置 ； 有效射源至表面距離

並列摘要

Purpose: The ESSD of three different types of Varian linear accelerators have been measured for electron beams in extended SSD treatment. The ESSD values were supplied the dosimetry calculation for routine patient treatments. According to the results, we compare and discuss the differences among the three different types of Varian linear accelerators.
Materials and Methods: The N.E. 0.03 c.c. parallel plate ionization chamber was installed on the chamber frame of Scanditronix RFA-300 3D water phantom and was connected to Ionex Dosemaster 2590 for dose measurement. The proximal electrode surface was placed at the depth of D(subscript max) to measure the delivered dose of each energy, each electron cone at SSD=100, 110, 120cm respectively. The ESSD of each electron cone and each energy was calculated according to the inverse-square-law using the data of SSD 100 and 120cm.
Results: The lower beam energies and the smaller electron cone, the ESSD become shorter. The same nominal energy, the larger cone size, the ESSD get more close to 100cm. The same electron cone size, the larger electron energy, the ESSD did not change regularly. The ESSD formed by MLC type electron cone varies smoothly. The scatter from Kapton type ion chamber is smaller than that from Mica type ones. Thus, the ESSD formed from the Kapton type machine is closed to 100cm.
Conclusion: The measured ESSDs of the three linear accelerators will be used to calculate the monitor units in the extend SSD electron beam treatment. From this study, for Varian linear accelerators, no any two machines got the same ESSD whether the same nominal energies or the same type of electron cones.

並列關鍵字

Radiation therapy ； Electron beam therapy ； Virtual source position ； Effective source-to-surface distance