Patient-specific CBCT Number Corrections for Cone Beam CT-based Dose Calculations

利用個別病患影像修正錐狀射束電腦斷層數值重建影像導引之劑量計算

李星怡(Hsing-Yi Lee) ； 杜佩芝(Pei-Chih Tu) ； 游佩潔(Pei-Chieh Yu) ； 雷德(Louis Tak Lui) ； 蕭世禎(Su-Zun Shaw) ； 吳錦榕(Chin-Jung Wu) ； 粘心華(Hsin-Hua Nien)

《放射治療與腫瘤學》 22卷3期 (2015/09) Pp. 199-209

https://doi.org/10.6316/TRO/201522(3)199

摘要

目的：撰寫程式修正錐狀射束電腦斷層數值，並於治療計劃系統計算此重建影像導引影像，以此評估此程式修正之重建影像用以計算病患每日真實接受劑量的可行性。材料與方法：在電腦斷層掃描定位影像與錐狀射束電腦斷層影像取得多處相同位置圈選區域之電腦斷層數值，如：空氣、組織、脂肪與骨頭。藉電腦斷層掃描定位影像之數值－密度對應表找出該圈選區域之密度，並依此密度重新建立電腦斷層掃描定位影像之數值－密度對應表與錐狀射束電腦斷層影像之數值－密度對應表兩者的線性迴歸方程式。錐狀射束電腦斷層影像之數值依據其線性迴歸方程式求得轉換吻合電腦斷層掃描定位影像線性迴歸方程式之數值。然而，錐狀射束影像之視野範圍（FOV）受到掃描條件所限制，部份組織因而未包含於錐狀射束影像之中。利用電腦斷層掃描定位影像補足錐狀射束影像視野範圍（FOV）外的影像以提供真實的衰減與散射條件。使用均質假體、人形非均質假體與十二位病患影像，透過此程式修正後之錐狀射束電腦斷層影像於治療計畫系統重新計算，以評估當日治療劑量與計畫規劃之準確性與差異。結果：經過治療計畫系統的計算，其兩者點劑量差異於6 MV結果為0.71%（SD=1.73%），10MV差異結果為0.49%（SD=1.39%）。平面Gamma index比對結果，6 MV通過率97.46%（SD= 4.08%），10 MV通過率 99.32%（SD=0.90%）。為求臨床應用驗證，挑選形變較小之病患，計畫靶體積D95%的劑量差異結果為0.48%（SD=1.19%），計畫靶體積的處方劑量包覆性差異為0.36%（SD =1.69%），計劃靶體積之一平面Gamma index比對結果通過率98.06%（SD=1.66%）。結論：此程式利用個別病患影像修正錐狀射束電腦斷層數值，用以重建病患每日真實接受劑量有其臨床評估參考的可行性。

關鍵字

錐狀射束電腦斷層 ； 影像導引放射治療

並列摘要

Background and Purpose: We develop an in-house program to correct computed tomography (CT) numbers of cone beam CT (CBCT) images and to recalculate modified CBCT images by using treatment planning system (TPS) software to evaluate actual patient doses. Material and Methods: Mean CT numbers were determined for planning CT and CBCT for the same selected regions of air, tissue, and bone. Based on CT-density tables for the planning CT, the density of CBCT could be determined for a selected volume. After generating linear regression equations for both the CT-density table and the CBCT-density table, the CT numbers for CBCT could be modified to fit the planning CT-density table. The field of view (FOV) boundaries in the corrected CBCT images were identified and merged with the planning CT images to provide real scatter and attenuation data. Images of 2 phantoms and 12 patients were used to verify the accuracy of corrected CBCT doses. Results: Based on Pinnacle TPS calculations, the differences in point dose between CBCT and CT images in the phantom study were 0.71% (SD=1.73%) at 6 MV, and 0.49% (SD=1.39%) at 10 MV. The 2D passing rates for a gamma index were 97.46% (SD=4.08%) at 6 MV, and 99.32% (SD=0.90%) at 10 MV. The dose difference of D95% was 0.48%±1.19%. The difference in planning target volume (PTV) coverage of the prescribed dose was 0.36%±1.69%, and the 2D passing rate without threshold was 98.06%±1.66%. Conclusion: Patient-specific correction of daily CBCT images can be used for reconstructing daily dose distribution and is practicable for radiotherapy treatment and clinical evaluations.

並列關鍵字

Cone beam CT (CBCT) ； Image-guided radiation therapy (IGRT)