螺旋刀治療系統(Helical Tomotherapy)放射治療給予劑量的方式是以獨特螺旋斷層式強度調控放射治療技術(TomoHelical-IMRT)傳遞劑量，在照射的過程中，治療床緩慢地前進，機頭(gantry)不停旋轉同時以多葉式準直儀之葉片不停地開合照射，形成強度調控的扇形射束。因此在執行電腦治療計劃最佳化疊代(iteration)計算時，有其 特殊參數設定，照野寬度(field width, FW)、調變因子數值(modulation factor, MF)與間距(pitch)，不同的組合設定 會對治療計畫的結果產生一定之影響。本研究依據鼻咽癌病例腫瘤位置，假想規劃治療靶體積與危急器官在侖道 假體(Rando phantom)電腦斷層影像上，設計出33個不同照野寬度(5 cm、2.5 cm、1 cm)與不同調變因子數值(1.0、 1.4、1.8、2.2、2.6、3.0、3.4、3.8、4.2、4.6、5.0)搭配組合設定之電腦斷層治療計劃，並且執行相同的權重與相同的最佳化疊代計算次數。結果顯示，照野寬度會影響治療時間，5 cm、2.5 cm 和1 cm 所需的照射平均時間分別為244 秒、439 秒和1025 秒。另外當調變因子數值越高，將造成機頭旋轉一圈速率小於15 秒/每圈，而增加治療時間。本研究結果分析顯示螺旋刀放射治療頭頸癌電腦治療計劃在劑量分布及治療時間之參數最佳化設定組合為照野寬度1公分與調變因子數值為2.6。而照野寬度越小所需的治療時間就越久。

Helical Tomotherapy (HT) is a novel technique of Helical intensity modulated radiation therapy (Helical-IMRT). The rotary motion of the gantry combined with the advancing motion of the table around the patient along a spiral track. HT can deliver highly conformal dose distributions and spare critical organs. However, field width (FW), modulation factor (MF) and pitch are parameters that influence the quality of treatment plans in HT. This study examines the relationship between these parameters and their influence on the quality of treatment plans. To prepare the treatment plan, CT scans of RANDO phantom were used. A target corresponding to a nasopharyngeal cancer and critical organs were all included in the scans. Finally, 33 plans with the same set of importance and penalty parameters were obtained, differing in the value of FW (1 cm, 2.5 cm and 5 cm) and MF (1.0, 1.4, 1.8, 2.2, 2.6, 3.0, 3.4, 3.8, 4.2, 4.6 and 5.0). The ratio between average treatment time with different field widths (5 cm, 2.5 cm and 1 cm) was 244, 439 and 1025 secs, respectively. The analysis showed optimal combinations of MF = 2.6 for a FW of 1 cm, but small field width will increase in treatment time.