利用游離腔、二極體及EBT3 軟片驗證Elekta Versa直線加速器之百分深度劑量與剖面劑量

Verification of PDD and dose profile of Elekta Versa through ion chamber, diode and EBT3 film

薛涵冰(Han-Ping Hsueh)

指導教授 ： 張力允

義守大學/電機資訊學院/資訊工程學系/碩士(2017年)

摘要

對於癌症病患，放射治療計畫在執行前或期間，必須驗證所給予的劑量是否正確，目前普遍做法是採用農夫型游離腔 (farmer chamber)量測治療計畫中某點的點劑量，然而，若要知曉治療的剖面劑量(dose profile)則會花費很多時間，或者可以使用其他較昂貴設備。EBT3 film(軟片)是一種輻射變色軟片，因和人體有效原子序相似且可自體顯影，只要搭配一台標準掃描器，經過劑量校正後，就可得知軟片上二維空間中每點的絕對劑量，故為量測dose profile一個很好的工具。
　　為因應單次高劑量的治療方式，直線加速器藉著使用無整平濾器(Flattening Filter Free, FFF)的模式提高劑量率，縮短治療時間。在FFF模式下，dose profile並非平坦曲線，因此本篇論文將使用semiflex chamber、EDGE detector和EBT3做dose profile的量測，找出真實且精準的dose profile，做為電腦治療計畫計算劑量的依據。
　　本實驗設計將軟片垂直放置於固態水假體(RW3)中，並讓軟片中間通過放射線射束中心，透過放置在軟片下方的farmer chamber量測絕對劑量，再搭配百分深度劑量PDD (Percentage Depth Dose)和Matlab軟體求得軟片的劑量擬合曲線公式。後取得軟片FFF模式下之dose profile和年度品質保證semiflex chamber及EDGE detector所量測的dose profile，相互做比較。
結果顯示，當EBT3的分析面積單位和EDGE detector的作用截面相似時，dose profile的輻射半影區量測值會和EDGE detector的量測結果相似，相反的，當EBT3的分析面積單位靠近semiflex chamber的作用截面大小時，量測dose profile的輻射半影區其量測值又會靠近semiflex chamber的量測結果。
　　由實驗得知EBT3在空間解析度擁有較彈性的選擇，可藉由調整分析面積單位大小，匹配於不同劑量量測工具之量測結果。並透過分析EBT3不同的分析面積單位和其所顯示的劑量不準確度，提供適當的分析面積單位讓後人做參考。

關鍵字

輻射變色軟片 ； 軟片 ； 剖面劑量

並列摘要

Prior and during cancer treatment, it is essential for the physicist to quantify the dosage of the radiation; this ensures the quality and proper execution of the treatment plan. The most common way today to measure the dosage is utilizing farmer chamber. The limitation of farmer chamber is that it provides dosage of a specific point; however, the beam dose profile is needed for the planning system, and the general measurement is either time-consuming or relatively expensive. In contrast, the Gafchromic EBT3 film can provide a quantitative dose profile in a 2D space. The EBT3 film has low energy dependence, similar effective atomic number to the human body, and self-developing; making it an ideal candidate for dose profile measurement.
To reduce the treatment time, treatment plans are often designed using high radio-dosage mode, the Flattening Filer Free (FFF ) Mode of Linac. In FFF Mode, the dose profile demonstrates non-linear curve along the axis away from central-axis. In this study, dose profiles measured through semi-flex chamber, EDGE detector and EBT3 were analyzed. The EBT3 films used for this study was setup at the center of a stacked RW3 water phantom (30x30x30 cm3) with the photon beam. Under that, additional RW3 with a Farmer chamber inserted in it were used. A dose fitting curve was fitted using PDD and Matlab to calibrate the EBT3. The dose profiles were also measured through a semi-flex chamber and an EDGE detector for comparison.
The penumbra of the measured dose profile through EBT3 film demonstrated less differences when comparing with that of the EDGE detector. But noticeable differences were observed when compared it to that of the semi-flex chamber. This study shows that EBT3 film is an excellent QA tool with accurate dosimetry, a good spatial resolution and tolerable dose uncertainty.

並列關鍵字

EBT3 ； film ； diode ； EDGE detector ； dose profile

參考文獻

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