研究目的： 比較三維順形放射治療技術（three dimension conformal radiation therapy, 3D-CRT）、強度調控放射治療（intensity modulation radiation therapy, IMRT）及體積調控弧形 放射治療（volumetric-modulated arc therapy, VMAT）三種治療技術利用器官等效劑量（organ equivalent dose, OED）分析建立二次癌症的風險評估模型，以得到額外絕對危險度（Excess absolute risk, EAR）。 材料與方法：選取2010 年3 月至2016 年2 月間接受全腦脊髓照射（craniospinal irradiation, CSI）治療的五位病患做為研究對象。規劃3D-CRT、IMRT 及VMAT 三種不同之治療計畫，將 治療計畫所得之劑量體積直方圖（dose-volume histogram, DVH）資料輸出，執行器官等效劑量 （OED）之參數計算，建立額外絕對危險度（EAR）之三種模型，用以評估因放射治療所誘發之 二次癌症機率。 結果：VMAT 較3D-CRT 及IMRT 顯著的減少特定危急器官之平均劑量。在額外絕對危險度 （EAR）評估結果中顯示，在全腦脊髓照射中引起小腸（small intestine）二次癌症之額外絕對危 險度為每萬人中大於50 人，其中以3D-CRT 之治療技術評估結果為最高；肺部（Lungs）二次癌 症之額外絕對危險度為每萬人中大於30 人，以VMAT 之治療技術評估結果為最高。在肝臟及口 腔二次癌症之額外絕對危險度為每萬人中小於10 人以內，機率極低。 結論：本研究為首篇針對全腦脊髓照射（CSI）三種不同之技術進行劑量學之差異比較，並建立台 灣本地接受CSI 放射治療患者二次癌症危險度參數，提供臨床治療規劃之二次癌症危險度參考指 標。

Purpose : To compare the secondary cancer risk (SCR) in cranio-spinal irradiation for three radiotherapy- planning techniques, namely, three dimension conformal radiation therapy (3D-CRT), intensitymodulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). The SCRmodel was established by usingthe organ equivalent dose (OED) to calculate the value of extra absolute risk (EAR). Materials & Methods : In total five patients were included who underwent cranio-spinal irradiation between March 2010 and February 2016 in our hospital. Three different treatment plans were created including 3D-CRT, IMRT and VMAT. The dose-volume histogram (DVH) obtained was used to calculate OED and SCR. Three SCR models established can be utilized to evaluate secondary cancer risks after cranio-spinal irradiation. Results : The highest EAR value in small intestine was greater than 50 for 3D-CRT; as well as in lung was greater than 30 (per 10,000/per year) for VMAT. However, the EAR values in liver and oral cavity were less than 10 (per 10,000/per year) for the three techniques. Conclusion : This is the first study to establish the EAR parameters and evaluation of SCRs by three radiotherapy-planning techniques in cranio-spinal irradiation in Taiwan.The result can be used as a clinical reference for evaluating secondary malignancy risks after radiation therapy.