目的：傳統肺部腫瘤的放射治療計畫中，多以增加照射範圍，來避免因呼吸運動的腫瘤位移所造成的幾何誤差(geometric miss)，但這仍無法確保腫瘤是否接受足夠劑量，反而可能因此增加正常組織的不當照射。本研究利用能記錄呼吸運動的四維電腦斷層掃描儀(4-Dimensional Computed Tomography, 4DCT)進行肺部腫瘤的模擬定位攝影，以4D CT中的最大密度投影(Maximum Intensity Projection, MIP)，來定義可視腫瘤體積(Gross Tumor Volume, GTV)及計畫靶體積(Planning Target Volume, PTV)，並與一般正常呼吸所得電腦斷層影像進行比較。材料與方法：在通過本院人體試驗委員會審核後，本研究取得肺癌或轉移性肺部腫瘤共12位病患的同意，進行4D CT模擬定位攝影。於正常呼吸的狀況下，首先進行傳統CT模擬定位攝影，之後再進行4D CT模擬定位攝影，用以製作MIP影像，最後比較分析GTV與PTV在這兩組影像中的差異。結果：正常呼吸影像中的GTV會小於MIP的GTV影像，而當依據影像特性加上各自安全距離(safe margins)成為PTV時，則正常呼吸影像中PTV會大於MIP的PTV影像(p<0.01)。而當腫瘤位置位於肺部carina以上，腫瘤位移情況明顯較位於carina以下的為小。若仔細檢視各病患肺內腫瘤的移動，12個案例中有3例出現正常呼吸時的PTV無法完全包覆MIP的PTV的情況，這三例腫瘤皆位於下肺葉，這代表若僅根據正常呼吸所規劃之治療，很有可能無法給予腫瘤足夠放射劑量。討論與結論：利用4D CT中MIP影像的功能來製作放射線治療計畫，可提升放射線治療的準確性，同時有機會減少PTV的體積大小，進而減少正常肺部組織照射的體積，降低因放射線造成嚴重副作用的可能性。

Purpose: The purpose of this study is to describe our management of the respiration-induced uncertainty by the Maximum Intensity Projection (MIP) of 4-dimensional (4D) CT.Material and Methods: Gross target volumes (GTVs) and planning target volumes (PTVs) were contoured on the images of normal-breathing CT and MIP of 4D CT for 12 patients with primary lung cancer or metastatic lung tumor. The volumes of GTVs and PTVs were compared between the two sets of images by using the paired t-test.Results: The GTVs of MIP images (mean=8.13±9.05 cm^3) were larger than those of normal breathing images (mean=4.74±5.89 cm^3, p=0.04). When applying different safe margins on GTVs to define their PTVs, the PTVs of MIP images (mean=25.28±20.38 cm^3) became smaller than those of normal breathing images (mean 75.04±36.49 cm^3, p=<0.01). Moreover, in 3 cases whose tumors were located in the lower lung, the PTVs of normal breathing images could not fully cover the GTVs of the MIP images. On the contrary, for tumors above the carina, the differences of target volumes between the two image sets were smaller.Discussion and Conclusion: 4D CT depicts the excursion of a lung tumor and increases the accuracy of the target definition. The 4D CT-based plans can reduce the target volumes to spare more normal tissues compared with conventional plans.