瓦里安碳纖維治療床之衰減及射束與治療床的幾何關係

The Study of Radiation Beam Attenuation Caused by Carbon Fiber Couch in Different Gantry Angle and Beam Geometry in Radiation Therapy

鄭因絢(Yin-Hsuan Cheng) ； 林坤生(Kun-Sheng Lin) ； 何志偉(Jr-Uei Ho) ； 何迦聖(Chia-Sheng Ho) ； 郭力誠(Li-Cheng Kuo) ； 陳筱萱(Shiau-Hsuan Chen) ； 陳文政(Wen-Cheng Chen) ； 陳妙芬(Miao-Feng Chen) ； 游浚彥(Chun-Yen Yu)

《放射治療與腫瘤學》 13卷1期 (2006/03) Pp. 47-56

https://doi.org/10.6316/TRO/200613(1)47

摘要

前言：電腦治療計劃主要是以電腦斷層掃描影像作為劑量計算及角度設計的基礎，但電腦斷層影像並未包含治療床，當治療照野經由治療床下方角度入射病患體內時，病息實際接受的劑量與電腦治療計劃會有所差異。因此，治療床對輻射所造成的衰減是必須評估的。
方法與材料：本研究是針對Varian 21EX直線加速器所配備之碳纖維治療床(Exact Couch)進行衰減測量。將游離腔套上直徑6公分的增建帽，置於治療床中心，射源－中心軸距離(source-axis distance, SAD)為100公分，分別使用6 MV及10 MV能量之光子以5×5平方公分，10×10平方公分，15×15平方公分照野照射。射束角度則是將旋轉臂(gantry)從0°至360°每隔5°作為入射角度，針對每個角度測量3次，每次照射劑量為50 MU。照射一圈後以旋轉臂0°為測量基準值，求出各個測量角度的讀值與旋轉臂0°讀值的衰減百分比。
結果：經實際測量後，射束中心軸未通過治療床及其支撐架者，與旋轉臂0°讀值相比之差異在0.5%左右；通過治療床但不經過支撐架者，與旋轉臂0°讀值相比之差異在1%之內；而通過治療床及其支撐架者，與旋轉臂0°讀值相比之差異有六處衰減超過10%，分別在97.5°、102.5°、120°、240°、257.5°、262.5°(IEC scale)。
結論：我們從測量數據中發現，共有六處產生較大的衰減，分別為兩邊的支撐架的上下緣及治療床邊緣所造成。射束經過治療床及支撐架而受到衰減的因素主要與治療床表面至治療中心的距離、治療照野大小及治療中心與治療床之中心距離有關。透過上述三個因素的數值，配合上三角函數與幾何關係，產生可推導出治療角度與治療床相互關係的方程式，藉由方程式便可找出射束照野不會通過支撐架的治療角度。

關鍵字

碳纖維治療床 ； 衰減 ； 支撐架

並列摘要

Purpose: The does calculation of all commercialized treatment planning computer system are based on Computerized Tomography image, but treatment room couch information is not included in CT images, therefore, the delivered does is literally different to the does distribution designed by treatment planning computer system. Especially when radiation beam incident posterially under the treatment couch, the phenomenal of radiation attenuation caused by treatment couch should be studied.
Material and Method: This study was to measure the attenuation caused by carbon fiber Exact couch in Varian 21EX Linear accelerator treatment machine, ion chamber was used to take the radiation does converted to reading with 6 cm build up cap at SAD 100 cm isocentrally setup. Field size was set to be 5 by 5 cm^2, 10 by 10 cm^2,15 by 15 cm^2 with different gantry angle from 0° to 360° in every 5° for 6 MV and 10 MV photon beam.
Results: According to the experimental measurement results, the electrometer chamber reading of those gantry angle which central axis did not penetrate the couch and the supporter compared to 0° gantry angle was within 0.5%, while the attenuation result was within 1% when the radiation penetrate through couch and exclude supporter, and there are six position whose attenuation results are exceed 10% there are at gantry angle 97.5°, 102.5°, 122.5°, 237.5°, 257.5°, 262.5° IEC scale.
Conclusion: The geometry of radiation beam and supporter were related to distance from treatment isocenter to couch surface in the vertical direction, field size, and shift of the couch to isocenter in the horizontal direction. We can get a gantry angle from the relation avoid the radiation to hit the supporter. Therefore, a set of equation was developed to check if the radiation beam penetrates the couch supporter or not.

並列關鍵字

Exact couch ； Attenuation ； Removable accessory rail