应用锥形束CT研究盆腔肿瘤放射治疗分次间及分次内的摆位误差

A Study of Interfractional and Intrafractional Setup Errors in Radiotherapy by Cone Beam CT (CBCT) for Pelvic Cancer

[目的]研究盆腔肿瘤放疗分次间及分次内的摆位误差,计算临床靶区(CTV)到计划靶区(PTV)的外放边界(MPTV)。[方法]应用ELEKTA Synergy IGRT直线加速器系统治疗盆腔肿瘤24例,通过锥形束CT(CBCT)影像技术获得患者左右(X)、头脚(Y)、前后(Z)方向线性摆位误差以及分别以X、Y、Z轴旋转形成相应的U、V、W旋转摆位误差,分析分次间、分次内的摆位误差,计算MPTV。[结果]24例患者共行365次首次摆位后CBCT扫描,系统误差(均数)±随机误差(标准差)在X、Y、Z方向上分别为(0.73±1.67)mm、(0.11±4.69)mm、(-1.77±2.60)mm,U、V、W方向上分别为(0.81°±1.11°、-0.01°±1.18°、0.39°±0.88°;纠正后摆位误差显著低于首次摆位后摆位误差(P<0.05),治疗后摆位误差较纠正后显著增加(P<0.05);纠正前X、Y、Z方向的MPTV分别为4.93mm、12.63mm、7.06mm,纠正后X、Y、Z方向的MPTV分别为1.25mm、2.43mm、1.67mm。[结论]盆腔肿瘤放疗时Y方向摆位误差最大,Z方向次之,X方向最小,旋转误差一般不超过3°;应用CBCT实施IGRT,可在线实时纠正分次间的摆位误差,提高放疗的精确度;应用CBCT引导放疗时,MPTV可缩小至所有方向均为3mm。

[Purpose] To analyze the interfractional and intrafractional setup errors in radiotherapy for pelvic cancer and to estimate the margin required for clinical target volume （CTV） to planning target volume （PTV）（Mprv）. [Methods] Twenty-four patients with pelvic cancer were treated with ELEKTA Synergy IGRT linear accelerator. Linear errors（X, Y, Z axis） and rotational errors（U, V, W） were obtained by cone beam CT （CBCT）. Interfractional and intrafractional setup errors and MPTV were analyzed. [Results] Twenty-four patients received a total of 365 CBCT scans after initial setup. The systemic＋random errors on X（left-right）,Y（superior-inferior）, Z（an- terior-posterior） directions were（0.73±1.67） mm, （0.11±4.69） mm, （-1.77±2.60）mm respectively, and those on U,V,W directions were （0.81°±1.11°）,（-0.01°±1.18°,（0.39°±0.88°） respectively. All the setup errors were corrected. Comparing to the initial setup errors, the re-positioning set- up errors decreased significantly （P〈0.05）. Setup errors after radiation delivery were increased significantly compared to the re-positioning setup errors （P〈0.05）. MPTV before re-positioning was 4.93mm in X axis, 12.63mm in Y axis,7.06mm in Zaxis. After re-positioning MPTV was 1.25mm, 2.43mm, 1.67mm in X, Y, Z axis, respectively. [ Conclusion] The spatial distributions of setup error is maximum in Y direction,moderate in Z direction,and minimum in X direction,and the most of rotational error was less than 3°. Real-time online correction of setup errors using CBCT can reduce the interfractional errors and improve the precise of radiotherapy in patients with pelvic cancer. With the use of CBCT, MPTV may reduce to 3mm in all directions.