图像引导鼻咽癌调强放疗技术和质量保征

Quality assurance and methods based on the image guided intensity modulated radiotherapy for nasopharyngeal carcinoma

目的:探讨图像引导鼻咽癌调强放射治疗技术和质量保证(QA)方法。方法:利用千伏锥形束CT(KVCBCT)引导15例初治鼻咽癌患者调强放射治疗,将KVCBCT得到的位置差异,推导得到靶区勾画CTVPTV的边界;以CT模体检验CBCT图像质量和等中心偏差;以矩阵电离室对调强计划进行剂量验证。结果:对15例鼻咽癌280次CBCT扫描中,3个方向偏差,X方向:0.55±1.03mm,Y方向:0.72±2.25mm,Z方向:0.42±1.14mm,3个方向小于2mm的偏差比例分别为86.3%、76.7%、83.8%;大于3mm偏差分别为5.9%、9.4%、6.3%。对KV-MV等中心验证,三个方向融合差值分别为0.2±0.3mm、0.4±0.3mm、-0.2±0.5mm;用矩阵电离室验证调强计划相对剂量,对于单野,Gamma值为93.2%-97.2%,对于整个计划Gamma值为95.0%-97.7%。绝对剂量验证主要是对等中心点、剂量均匀区、高剂量区、较低剂量区、高梯度区选择5个点进行检测,百分偏差范围为-3.7%-4%。结论:图像引导鼻咽癌调强放疗,可以减少摆位引起的摆位误差,并且通过在线的修正可以提高靶区剂量的准确,也可减少CTV-PTV的边界,从而减少正常器官的剂量。保证图像引导部分的成像质量和机械精度是图像引导放疗的关键;而计划剂量的验证是所有治疗的基础。

Objective： To discuss the aspects of quality assurance and methods based on the image guided intensity modulated radiotherapy （ IG - IMRT） for nasopharyngeal carcinoma （NPC）. Methods： Fifteen patients of NPC treated with IG- IMRT were investigated. The online kilovoltage cone- beam CT （KVCBCT） scans,rigid image registration, set - up error was gained for 280 sets before radiotherapy. From the set - up error of three directions, we got margins form clinical targets volume （CTV） to planning targets volume （PTV）. In order to provide more accurate isocenter of the KVCBCT and the linear accelerator,we developed CT phantom to gain images quality of CBCT and ensure KV -MV center. Results： All 280 sets of CBCT image were analyzed for 15 patients. Under the condition of non - correction,the setup isocenter errors were O. 55 ± 1.03mm,0.72 ±2.25mm,0.42± 1.14mm in left - fight,superior - inferior and anterior - posterior directions respectively. The errors of less than 2mm were 86.3% ,76.7% ,83.8% in the three directions respectively,the errors of more than 3mm were 5.9% ,9.4% ,6.3%. We developed Gamma value and absolute dose value （ 〈4% ） to evaluate the accuracy of IMRT plans based on 2D ionization chambers array. The Gamma value of each beam of the plan was 93.2% -97.2% ,and the value was 95.0% -97.7% in each plan. The absolute dose value was chosen from five points among the high dose region,low dose region,isocenter, high gratitude dose religion and dose flatness region. All the dose deviation was in -3.7% -4%. Conclusion： We can gain the margins of CTV -PTV from IG- IMRT,and reduce dose of the risk organs,which will reduce normal tissues treatment - related toxicity. The key role to improve quality assurance of CBCT and the key foundation is how to ensure the dose accuracy of IMRT plans.