放射治療在癌症治療扮演重要的角色，而治療技術的進步不僅提高腫瘤控制率。特別是更可減少正常組織的傷害，可藉由強度調控放射治療（Intensity modulated radiation therapy，IMRT）比傳統治療大大地減少非必要照射組織的區域，以鼻咽癌病患來說明顯可減少對唾液腺的傷害，以肺癌及食道癌病患來說明顯可減少對肺部的傷害，以胰臟癌病患來說明顯可減少對小腸的傷害。近年來發展的全體積弧形強度調控放射治療（Volumetric modulated arc therapy，VMAT）結合旋轉角度和強度調控的優勢，對於腫瘤區域鄰近危及器官的病人在劑量上可達到更順形的需求，提昇治療計畫品質 但是呼吸引起的體內移動又會使順形及劑量準確打折扣。
本研究利用5位已使用VMAT技術治療的胰臟癌病患，以相同的影像、靶區和危急器官等輪廓定義下，重新以分段VMAT設計新的治療計畫，進行回溯性劑量學比較。以等劑量曲線（Isodose curve）、劑量-體積直方圖（Dose-volume histogram，DVH），和實際執行假體劑量驗證等做為評估。
在結果顯示，治療計劃皆可在靶區有相同的劑量包覆；執行假體劑量驗證方面，損失的劑量及位置準確度極其些微，Gama Index (3%, 3mm) 皆在95% 以上。顯見在直線加速器執行能力是可達到的。
如將呼吸因素考慮進去，分段VMAT比不分段VMAT則有較佳的劑量均勻性及順形度；在執行效率部分，VMAT比IMRT可以較短的時間及較少的監控單位完成治療，減少治療時間過長造成病患的移動及不適，尤其在主動式呼吸協調治療這種需要較長時間完成的病例更合適。

Abstract
Radiation therapy plays an important role in most of cancer treatment. Its advances in treatment technology not only improve tumor control probability (TCP), but also lower normal tissue complication probability (NTCP). Intensity modulated radiation therapy (IMRT) has greatly mitigated the shortcomings of traditional radiotherapy by reducing the damage to normal tissues. A newly developed technique, volumetric modulated arc therapy (VMAT), combines inverse planning of intensity modulation and a full 360° radiation beam direction resulting in better dosage conformity for the cancer treatment with lower doses to the surrounding organs at risk.
We programmably divided long-arc volumetric modulated arc therapy (VMAT) into split short arcs, each taking less than 30 s for respiratory coordination. The VMAT plans of five pancreatic cancer patients were modified; the short-arc plans had negligible dose differences and satisfied the 3%/3-mm gamma index on a MapCHECK-2 device.
The study results demonstrated that with the same target coverage, radiation therapy using segmented VMAT could provide better homogeneity. However, whole arc VMAT attained significantly easy planning because of fast iteration and calculation time. In terms of normal tissue protection, where segmented VMAT has their own advantages due to adopted active breathing coordinate provide better dose sparing to the OARs. VMAT outperformed segmented VMAT by lower treatment time and thereby relieve the patients from discomfort. Segmented VMAT avoid body motion due to prolonged treatment time.
Key words: intensity modulation radiation therapy, volumetric modulated arc therapy

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