目的：銳速刀（RapidArc）放射治療是利用旋轉臂（gantry）在旋轉的同時，藉由改變多葉式準 直儀（multi-leaves collimator；MLC）葉片的位置、照野內的輻射劑量率以及gantry 旋轉速度， 以達到體積調控弧形放射治療的目的。本研究的目的主要探討使用銳速刀技術作計畫優化時，不 同劑量率設定造成計畫執行時劑量率變化及機頭旋轉速度的相互影響關係。且探討其對電腦治療 計劃品質及計畫品保結果的影響，希望找出在做計畫優化時，相對適當的機器劑量率條件設定。 材料與方法：本研究採回溯性方式選取本科患者資料庫中的頭頸癌3 名、食道癌患者5 名及攝護 腺癌患者5 名總共13 名患者。進行銳速刀治療計劃的優化過程時，劑量率設定為100、200、 300、400、500 及600 MU/min 等六種不同劑量率重新執行治療計畫優化。所有患者在進行銳 速刀治療計劃的優化過程中，對治療計畫靶體積（planning target volume；PTV）和各危急器官 （organ at risk；OAR）使用相同的劑量限制（dose constraint）及計畫優化優先權（optimization priority）以減少影響結果的變異性。評估劑量率設定各計劃在腫瘤均勻性、順形度以及危急器官 所接受的劑量，且比較MU 數、射束給予時間以及計劃品保結果。 結果：本研究結果得知受限於樣本數不足並無相當的證據或趨勢顯示治療計畫品質與不同劑量率 的相關性；設定高劑量率在計畫優化時會以最大機器旋轉速率為主要考量，因此劑量輸出變化與 機器旋轉速度相互調節的彈性較大；旋轉角度圈數越多，可調整空間也較多；高劑量率縮短較多 的治療時間，但不絕對局限於最高劑量率（600 MU/min）時。 結論：在執行銳速刀放射治療技術時，雖然計畫優化演算法本身的複雜性且受限於病患數不足， 但仍可知高劑量率有好的機器旋轉速度與劑量率相互調整的彈性及在減少治療時間上有相當的優 勢。

Purpose : RapidArc is the latest VMAT technique of radiotherapy delivery, varying multi-leaves collimator (MLC) shape, dose rate and gantry speed simultaneously during gantry rotation. The purpose of this study is to evaluate the interplay of dose rate setting and gantry speed during RapidArc optimization, and analyzing treatment planning and plan verify results to find out the most suitable dose rate setting. Materials and Methods : Thirteen patients included three head and neck, five esophageal and prostate patients were selected to re-optimize the treatment plan of RapidArc by setting six different dose rate (100, 200, 300, 400, 500 and 600 MU/min). All re-optimized plans used the same dose constraint and priority for planned target volume and critical organ. Compare the results of the homogeneity index (HI) and conformal index (CI) in the tumor, the dose received by critical organs, monitor unit numbers, delivery time and plan verified by setting different dose rate. Result : All plans results showed that there was no evidence and trend toward improved plan quality for clinical case was observed with any dose rate. When setting high dose rate, gantry maximal speed was the main consideration then it was more flexible in modulating gantry speed and dose rate and using more arcs in plan also did. The main advantage of setting high dose rate was saving total treatment time. But not mean that it must be setting the maximum dose rate (600 MU/min). Conclusion : RapidArc plan was complex and limited by amounts of cases in this study, but high dose rate had high dynamic range in gantry speed modulation. No doubt high dose rate could reduce total treatment time.