直線加速器是目前放射線治療最常見的設備，現今醫用直線加速器治療室的屏蔽設計主要參考NCRP-151報告。本研究有二大主要目標：(1)基於NCRP-151報告所建議的經驗公式，設計一套EXCEL試算表來協助使用者執行醫用直線加速器治療室的屏蔽計算，方便使用並避免人為失誤；(2)檢驗NCRP-151報告之經驗公式的準確度，利用蒙地卡羅法有系統地比較兩種方法估計結果的差異。
要嚴謹比較NCRP-151與蒙地卡羅法的差異是不容易的，因為NCRP-151經驗公式的射源是正規化到水箱假體內的劑量，而蒙地卡羅法的射源來自加速器輸出的電子數目。為探究NCRP-151準確性，本研究建立一組可用蒙地卡羅模擬的直線加速器機頭模型，透過射束品質及機頭洩漏比值的驗證，確立此模型符合NCRP-151所描述條件，再將之套入整體治療室屏蔽模型，藉此評估屏蔽內外的輻射場特性與劑量率。本研究設定18 MV能量的直線加速器，在同樣幾何與材料的典型直線加速器治療室及照射條件下，詳細比對NCRP-151與蒙地卡羅模擬的預測。研究結果顯示在主屏蔽外NCRP-151預測的劑量率低估約2.5倍，次屏蔽、門內及門外劑量率NCRP-151則高估了約10倍，最可能的差異來源為簡化的機頭組件，透過調整靶及整平濾器的幾何厚度可改變原本在屏蔽外劑量率低/高估的傾向。建議可從設備廠商獲取機頭機件詳細資訊來執行模擬，或取得百分深度劑量曲線100%最高點處實際量測絕對值來做擬合，互相驗證後進一步判別情勢(本研究只針對18 MV的直線加速器進行比較，並未針對10或6 MV的機型進行類似探討)。另外，本研究採用的蒙地卡羅程式為PHITS，有鑑於PHITS在台灣並不常見，比較缺乏使用經驗，為確保PHITS程式使用的正確性，本研究亦進行PHITS與MCNP6程式的比對，結果顯示二者有相當一致的預測。

Linear accelerators are currently the most common equipment for radiation therapy. Nowadays, the shielding design of medical linear accelerator treatment rooms is mainly according to NCRP-151 recommendation. There are two main goals for this study：(1)Design an Excel spreadsheet based on NCRP-151 to assist users in performing shield calculations for medical linear accelerator treatment rooms, making them easy to use and avoiding human error；(2) Examine the accuracy of the empirical formula in NCRP-151, and use the Monte Carlo method to systematically compare the differences between the two methods.
It is not easy to compare the differences between NCRP-151 and Monte Carlo methods. Because the source of the NCRP-151 empirical formula is the dose normalized into the water phantom, and the source of the Monte Carlo method is the number of electrons in the accelerator head. This study establishes a linear accelerator head model which can be simulated by Monte Carlo to examine the accuracy of NCRP-151. This model was established to meet the description in NCRP-151 by verification of the beam quality and leakage ratio. The model is then incorporated into the treatment room to evaluate the radiation field characteristics and dose rate inside and outside the shield. This study set the linear accelerator head model with 18 MV energy. In a typical linear accelerator treatment room of the same geometry、material and under the same irradiation conditions, making a comparison of NCRP-151 results and Monte Carlo simulation predictions. The results show that the dose rate predicted by NCRP-151 outside the primary barrier is about 2.5 times underestimated, and the dose rates outside the secondary barriers、inside and outside the door are overestimated about 10 times. The most likely cause of the difference is the linear accelerator head model assembly. By adjusting the geometric thickness of the target and the flattening filter, the trend of under/overestimation of dose rates outside the shield can be changed. It is suggested that detailed information of the linear accelerator head and assemblies can be obtained from the equipment manufacturer to perform the simulation, or obtain the actual measured absolute value at the 100% highest point of the percent depth dose curve for fitting, and the situation can be further examined after mutual verification (This study only for 18 MV linear accelerators, and does not discuss for 10 or 6 MV models.) Besides, the Monte Carlo program used in this study is PHITS. Considering PHITS is not common in Taiwan and lacks experience in using it, this study also compares the PHITS and MCNP6 programs to ensure the correct use of the PHITS program, and the results show fairly consistent predictions.

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