前言:為減少直腸肛門癌病患接受放射線治療時，因病人固定性不佳而造成腫瘤治療劑量不足或正常組織接受劑量過高的情形。我們採用新的治療中心設定方法，藉由驗證片及模擬攝影片的比較及測量，測定此一方法之精確性。

主法:病人以仰臥姿勢，除頭枕及腳踩固定器外不使用其他的固定材料。病人於電腦斷層模擬攝影峙，將治療中心固定在由治療床表面往腹部7公分的高度。治療中心定好之後，依照雷射投影在病人身上的位置做記號，並貼上X光可顯影之膠管，在電腦斷層模擬攝影片上顯示治療中心o第一;欠治療時拍下驗證片，和模撥攝影片比較後，測量兩者的誤差，包括位移及旋轉角度的差異。

結果:頭腳方向的平均位移為1.3± 3.3 mm '左右方向為1.9± 3.8 mm '前後方向為0.2± 2.0 mm。冠狀面平均能轉角度為0.76 ± 2.84度，矢狀面為0.52 ± 0.76度，橫斷面為0.04 ± 0.33 度。

結論:固定骨盆腔的後前治療深度是一個可靠的治療方式，在各方向的位移幾乎都少於5mm'其中雖然以冠狀面的旋轉誤差較大，但是仍在良好的範團內，因此、此種定位治療法是簡單且可靠的方式。

Purpose: To improve the reproducibility of irradiated volume in treating rectal cancer patients with fractionations， we have developed a new isocenter determination method and tested its accuracy by comp位ison of portal and simulation films.

Materials and Methods: Patients were placed in the supin巴position with only a head restraint and feet immobilizer， During Computerized Tomography (CT)， the isocenter was fixed at 7cm 台。mth巴bedSUfface in the direction of the abdomen， after which the position was marked on the patient's body by means of laser projection. A rubber tube was attached for visualization on x-ray films. Thβisocenter was thus indicated on the CT scan. During the first treatment， a portal film was made for comparison with the simulation film， and displacement and rotation errors of the two were measur吋.

Results:Mean head-to-toe displacement was 1.3± 3.3 mm， left-right displacement was 1.9± 3.8 mm and anteroposterior displacement was 0.2 ± 2.0 mm. The mean coronal rotation was 0.76 ± 2.84°， sagittal rotation was 0.52 ± 0.76° and transverse rotation was 0.04 ± 0.33°.

Conclusion:Stabilizing the position of the pelvis on the treatm巳nt couch is a reliable method for treatment. Displacement in almost all dir即tions was less than 5mm. The e訂or for coronal rotation was slightly greater， but nonetheless within ac臼ptable range. Therefore，this type of treatment localization method is a simple and reliable method.