Evaluation of the Dose Response of Gafchromic EBT Film for High Energy Photon Beams

評估EBT型自顯影軟片在高能光子下的劑量反應

梁永昌(Henry W.C. Leung) ； 何秀雯(Hsiu-Wen Ho) ； 李淞維(Sung-Wei Li) ； 羅素花(Su-Hua Lo) ； 胡尹薰(Yin-Hsun Hu) ； 林立青(Lih-Ching Li)

《放射治療與腫瘤學》 14卷3期 (2007/09) Pp. 189-196

https://doi.org/10.6316/TRO/200714(3)189

摘要

目的：自顯影底片近年來已被廣泛用於醫學物理領域中，它具有許多優點包含良好的空間解析度、相當低的能譜敏感度、接近組織等效、不需特別的洗片處理即可計讀。EBT型自顯影底片是ISP的最新產品，主要適用於強度調控放射線治療的劑量驗證。本篇研究的目的即在探討EBT型自顯影底片對臨床使用的高能光子之劑量反應。
材料與方法：本實驗中，以ELEKTA Precise LINAC作輻射源，將EBT型自顯影底片分別曝露在不同的光子能量、劑量率、分次及室內燈光下。在曝露後使用Radlink Laserport 16 scanner及Epson Perfection 4990 photo scanner，分別以紅光、藍光、綠光及灰階作為光源進行計讀。以計讀結果建立個別高能光子的校正曲線，並評估隨時間改變的顏色變化。
結果：結果顯示無論在不同分次或光子能量下，EBT型自顯影底片並無顯著性的差異存在。當劑量小於4 Gy時，以紅光為光源的Epson Perfection 4990 photo scanner有最高的劑量反應；而當劑量大於4 Gy時，則以綠光為光源的Epson Perfection 4990 photo scanner有最高的劑量反應。而Radlink Laserport 16 scanner無論在劑量大於或小於4 Gy情形下，反應皆為最低。軟片曝露後的光密度變化顯示，在曝露後1至5天有些微的增加；在第6天出現明顯的減少現象；在7至14天間呈現穩定的狀況。連續在室內燈光下曝露24小時會導致光密度增加0.117，約等於接受到10cGy的輻射曝露量。
結論：EBT型自顯影底片是一種適用於放射治療的新型劑量計，我們的結果顯示，它並沒有顯著性的分次依存性，而且在4MV、6MV及10MV並無明顯能量依存性存在。但是它對於室內燈光的不敏感性卻不如我們所預期的好。雖然如此，它仍適合作為臨床劑量計使用。我們建議EBT型自顯影底片在曝光後至少應等24小時再行計讀，計讀時應以紅光做光源以期有較好的反應。而軟片在不使用的情形下應儘量至於暗處以避免不必要的干擾產生。

關鍵字

EBT型自顯影底片 ； 遠端照射 ； 曝露後的光密度變化

並列摘要

Purpose: Radiochromic film has gained increasing popularity in medical applications in recent years. It has several advantageous features, including high spatial resolution, relatively low spectral sensitivity, near tissue equivalence, and it requires no special development procedure. This study aimed to evaluate the response of new type Gafchromic EBT film for high energy photon beams in clinical use.
Material and Methods: The ISP Gafchromic EBT film (prototype A) and ELEKTA Precise LINAC were used in this study. Gafchromic EBT films were exposed to radiation of difference energies, dose rates, fractionation and to the ambient light respectively. After completely being exposed, we scanned the film with Radlink Laserport 16 scanner and Epson Perfection 4990 photo scanner in red, blue, green channel and gray scale respectively. The post-exposure density growth with varied time was also evaluated. The calibration curves for high energy photon beams were established.
Results: The results showed that the effect of dose fractionation was insignificant, and there is no significant energy dependence on 4MV, 6MV and 10MV photon beams. The response using red light source of Epson scanner against corresponding delivered dose is higher than that of the green, blue channel, gray scale and Radlink LaserPor scanner at doses below 4 Gy. The gray scale model showed the lowest response in all scanners. It is interesting that the green light source would have the highest response when the dose was greater than 4 Gy. The post-exposure density growth test showed a slight increase between 1st to 5th day, at sixth day, an obvious decrease occur, then became stable between 7th to 14th day. However, the net OD was nonlinear with time and dose, continuous exposure to the room light for 24 hours resulted in a density change of about 0.117, which is equivalent to a radiation exposure of about 10 cGy.
Conclusion: Gafchromic EBT film is a new type of film for radiotherapy dosimetry. Its response to different dose rates and dose fractionations were insignificant, but theresults of the effect for room light exposure were not as good ad expected. However, it is still a reliable dosimeter for clinical dosimetry. Our suggestion is to read the film at least 24 hours after irradiation, and scan the films with color scanner in red channel to obtain better response. Furthermore, the film should be kept properly when not in use to avoid other confoundedly influences.

並列關鍵字

EBT film ； External beam ； Post-exposed density growth