应用非每日锥形束CT校位减少摆位误差对鼻咽癌调强放疗剂量影响

目的 探讨应用非每日千伏级锥形束CT(KVCBCT)校位能否改善摆位误差对鼻咽癌调强放疗(IMRT)剂量分布影响。方法 对14例行根治性IMRT的鼻咽癌患者治疗开始后连续5次用KVCBCT检测摆位误差,并将其均值作为系统误差预测值,若其＞1.5 mm则在第6次离线校位。假设通过移床能完全校正系统误差,那么从第6次起实际各方向摆位误差值加上离线校位值可得到未行校位时的摆位误差值,在治疗计划系统中通过等中心移位重新计算剂量来模拟应用非每日校位策略前后摆位误差所致的剂量变化。结果 对10例系统误差预测值＞1.5 mm者摆位误差明显降低了靶区剂量:98％大体肿瘤体积(GTV)所接受剂量(GTV-D98)平均减少3.8Gy(Z=-2.81,P=0.005),原发灶临床靶体积(CTVns) D95( CTVns-D95)平均减少4.8Gy(Z=-1.96,P=0.050),高危CTV1 -D95平均减少1.0Gy(Z=-2.82,P=0.005),低危CTV2-D95减少不明显(Z=-0.13,P=0.900)。应用非每日校位后明显减少了摆位误差的三维方向位移总量,均值从3.6 mm减少为2.6mm(t=2.00,P=0.000),GTV-D98平均增加3.8 Gy(Z=-2.70,P=0.007),CTVns-D95平均增加5.0Gy(Z=-2.15,P=0.030),CTV1 -D95平均增加0.9Gy(Z=-2.80,P=0.005),减少了危及器官剂量增加＞3％、5％患者比例。结论 应用非每日KVCBCT校位能有效减少摆位误差对鼻咽癌IMRT剂量分布的不利影响。

Using non-daily cone-beam computed tomography reduce dosimetric effect of set-up errors in intensity-modulated radiotherapy for nasopharyngeal cancer

Objective To assess the impact of set-up correction on dosimetry using non-daily kilovolt cone-beam computed tomography (KVCBCT) for nasopharyngeal cancer patients treated with intensitymodulated radiotherapy (IMRT). Methods The mean shift values from 14 nasopharyngeal cancer patients received KVCBCT scans during the first 5 treatment fractions were calculated as prediction of systemic set-up errors and used for off-line correction at 1.5 mm threshold level. Presumed that the systemic errors can be corrected by moving couch without residual errors, the pre-correction set-up errors in the remaining fractions were the sum of actual set-up errors and predicted errors. The dosimetric effects of non-daily protocol were simulated in the planning system and analyzed with physical dose parameters in 14 IMRT plans. Results In 10 patients with predicted systemic errors ＞ 1.5 mm, target dose was reduced significantly. The mean reduction of GTV-D98 ( dose received by 98％ of the volume of GTV ), CTVnx-D95 ( dose received by 95％ of the volume of CTVnx ), CTV1 -D98 ( dose received by 98％ of the volume CTV1 ) were 3. 8 Gy ( Z =- 2. 81,P =0. 005 ) ,4. 8 Gy ( Z =- 1.96, P =0. 050 ), 1.0 Gy ( Z =- 2. 82, P =0. 005 ), respectively. The effect on dose to CTV2 was much less. After correction, mean 3D vector positioning errors was reduced from 3. 6mm to 2. 3 mm (t =2.00,P =0. 000). After correction, the dose led to increase in GTV-D98, CTVns-D95,CTV1-D95 was 3.8 Gy (t=-2. 70,P=0.007),5.0 Gy (t =-2. 15,P=0.030),0.9 Gy (Z=-2.80,P=0. 005 ) respectively, and reduced the dose deviation greater than 3％ or 5％ for organs at risk.Conclusion Non-daily KVCBCT correction reduced dosimetric effect of set-up errors in IMRT for nasopharyngeal cancer patients.