- 學位論文
比較有無射束整平濾片技術於肺癌立體定位放射治療之皮表劑量差異
Comparison of Skin Surface Dose in Lung Cancer Treatment through Stereotactic Body Radiation Therapy by Using Flattening Filter and Flattening Filter Free Technique
摘要
目的：文獻指出，放射線治療肺癌的新技術立體定位放射治療對腫瘤的控制率較傳統的放射治療來的好。近年來無整平濾片(Flattening Filter Free，FFF)受到越來越多的注目，此技術是在治療的時候，將整平濾片從直線加速的機頭中移除，如此一來可以大幅度的提升劑量率縮短治療所需要的時間。但無整平濾片技術也有一些劑量學上的疑慮，首先移除了整平濾片，會讓射束缺少射束硬化而使得射束的平均能量降低，因此很容易提高皮膚的劑量。另一方面，正因缺少了整平濾片會降低射束通過濾片時所產生的散射劑量，而讓皮膚表面的劑量降低。因此本研究的目的是在於探討立體定位放射治療時，使用無整平濾片技術或是整平濾片技術，其照射範圍內皮膚劑量是否有明顯的差異。 材料與方法：本實驗使用Elekta Infinity™ 直線加速器，6 MV級光子射束及6 MV級無射束整平濾片光子射束作為標準射源。利用光激發劑量計(Optically Stimulated Luminescent Dosimeters，OSLDs)來進行立體定位放射治療皮表劑量測量。首先利用農夫型游離腔(Farmer-type ion chamber)來校正20顆OSLD並建立劑量校正曲線，再利用平行板游離腔 (Plane-Parallel ion chamber)驗證劑量校正曲線及進行表面劑量的量測量測試，最後再利用Rando phantom進行立體定位放射治療實際測量，在射束會經過的路徑上將OSLD黏貼在假體表面之後進行照射並測量照射時間，最後比較有無射束整平濾片技術其皮表劑量與照射時間的差異。 結果：完成OSLD的劑量校正曲線之後與平行板游離腔同時進行表面劑量測量測試時發現，FFF的表面劑量確實比FF來的大，但是會隨著增建深度增加、照野大小增加而使得兩者的表面劑量差異隨之減少。而在Rando phantom實測立體定位放射治療技術皮表劑量的結果發現，在射束經過的路徑上某些測量點FFF的皮表劑量確實會高於FF，單點劑量最高的位置 (4CR) OSLD測量FFF大於FF 24.8%、治療計劃系統(Treatment Planning System，TPS)計算FFF大於FF 14.2%。但是並非所有的測量點皆是FFF高於FF，並且將所有測量點劑量加總平均之後會發現兩者吸收劑量差異: OSLD量測的為0.5%而TPS計算的為1%。將FFF與FF的皮表劑量與皮下2 cm所得到的劑量比較之後，發現14個布點中，FFF相對劑量大於FF的只有7個點，比例為50%，顯示有無整平濾片並無明顯差異。照射時間FF技術平均為191.1秒、FFF技術平均為113.5秒，顯示FFF技術可以減少40.6%的照射時間。 結論與討論：測量結果發現在單一照野量測上FFF的皮表劑量比FF來的高，但實施立體定位放射治療時，劑量經過平均分散之後會發現兩者的皮表劑量及相對皮表劑量並無明顯差異，這樣是否會造成嚴重的皮膚反應仍需要臨床的追蹤檢查。FFF技術確實可以因為劑量率的提升而大幅縮短治療所需時間，因此臨床宜選用搭配無射束整平濾片技術的立體定位放射治療，如此能減少病人在治療中移動的可能性、增加定位和治療的準確度而提升腫瘤的控制率。
並列摘要
Purpose：Sterotatic Body Radiotherapy (SBRT) for lung cancer treatments has been proved that it has better local control rate than conventional radiotherapy. Recently, Flattening Filter Free (FFF) technique was highlighted for clinical studies, which is a new technique through removing the flattening filter from a linac treatment head to enhance the dose rate and reduce the beam-on time. However, the treatment with FFF was doubted to increase skin surface dose due to its softer energy specta, but on the contrary, it could reduce that dose due to less scatters from the gantry head. The purpose of this study was then to measure and compare the skin surface doses for the SBRT with FFF or Flattening Filter (FF) technique. Methods and materials：The linear accelerator, Elekta Infinity™ with 6 MV FF and 6 MV FFF photon beams, was used in this study. With FFF or FF technique for SBRT, twenty Optically Stimulated Luminescent Dosimeters (OSLDs) were used to measure the skin surface doses. All OSLDs were calibrated through a Farmer-type ion chamber and each calibration curves were established. The calibration was verified through the surface dose measurements by comparing that measured by a Plane-Parallel ion chamber. A Rando phantom was scanned by a CT sim and the treatment plannings of the SBRT with FFF and FF techniques were done. When performing the two techniques on the Rando phantom, the skin surface doses were measured with the calibrated OSLDs. Later on the surface doses and beam on times of the two techiniques were compared . Result： The verification tests demonstrated that the surface doses measured through the FFF technique were higher than that measured through the FF technique, but the differences were less for deeper depths and larger field sizes. The skin surface dose measurements through the Rando phantom show that the highest difference of skin surface dose using the FFF technique was 24.8% or 14.2% higher than that using the FF technique, measured by the OSLD or calculated by the treatment planning system (TPS), respectively. However, the difference of all summed surface doses between the two techiniques was 0.5% or 1% through the measurement of the OSLD or the calculation of the TPS, respectively. Comparing the relative dose, the surface dose divided by the dose at the depth of 2 cm, that of 7 out of the 14 test points demonstrated higher value by using the FFF technique. The mean beam on time using the FF technique and FFF were 191.1 sec and 113.5 sec, respectively. The beam on time would be 40% reduced, if the FFF technique was chosen. Conclusion： For the single field irradiation, the skin surface dose is relatively higher if using the FFF tecknique. However, the difference is not that clear, if those were measured on the Rando phantom by the irradiation from SBRT. That is probably the skin doses were averaged out due to the many incident directions. In clinics, the skin reaction of patient shouldbe followed up, if the SBRT was performed. Finally, the treatment by using the FFF tecknique can significantly reduced the beam-on time due to its high dose rate without causing severe skin problem. Therefore, the SBRT with FFF technique was suggested to be performed for the routine clinical treatment of lung cancerto reduce the error of patient movement and improve tumor local control rates.
參考文獻
延伸閱讀
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