鼻咽癌后装腔内放疗个体化施源器剂量学分析

Dosimetric analysis of custom-made intracavitary applicator for nasopharyngeal carcinoma treated with high dose rate brachytherapy

目的:评估热塑面罩固定鼻咽个体化施源器剂量的准确性及其应用价值。方法:选择10例后装放疗患者,分别制作鼻咽个体化施源器并使用面罩固定。每次治疗前均行CT扫描,并在计划系统中重建出施源器位置,获取铱源驻留位点坐标,通过与计划CT比较,计算出施源器位移偏差,进而评估后装治疗剂量的准确性。结果:非面罩固定最远端源驻留位点(P1)平均偏差在X、Y、Z三个方向为(1.62±0.24)、(2.32±0.22)和(2.50±0.28)mm,近端(P5)为(1.13±0.11)、(1.22±0.20)和(1.80±0.15)mm。经面罩固定后个体化施源器最远端源驻留位点(P1)平均位移则分别为(0.82±0.06)、(0.70±0.07)和(0.94±0.08)mm,近端(P5)为(0.70±0.02)、(0.46±0.01)和(0.60±0.03)mm。面罩固定施源器位移偏差导致的感兴趣点(源旁2cm)剂量误差<1.2%。结论:使用热塑面罩固定个体化施源器剂量准确,适合鼻咽癌腔内后装治疗。

OBJECTIVE: To evaluate the dosimetric accuracy of a custom-made intracavitary applicator and explore its use in high dose rate(HDR) brachytherapy of nasopharyngeal carcinoma.METHODS: Ten nasopharyngeal carcinoma patients treated with HDR intracaviatry brachytherapy were selected,each with a custom-made nasopharyngeal applicator.The patients were immobilized on supine position with individual head masks.CT scan was performed during treatment planning and before each treatment fraction.The nasopharyngeal applicator was reconstructed in PLATO brachytherapy planning system(BPS),making it possible to acquire three dimensional coordinates of source dwell positions.The position error of the applicator during the whole treatment sessions was thus assessed by comparing the coordinates of each source position in subsequent CTs with planning CT.The difference of immobilization with head mask vs.without mask was analyzed by using SPSS software.The potential dose error due to position inaccuracy was simulated in PLATO BPS.RESULTS:For patients without head immobilization mask,the mean position error of the distal end of the applicator was(1.62±0.24),(2.32±0.22) and(2.50±0.28)mm for P1 point,(1.13±0.11),(1.22±0.20) and(1.80±0.15) mm for P5 point,in X,Y,and Z direction.For patients immobilized with head mask,the corresponding displacement of the applicator was on average of(0.82±0.06),(0.70±0.07) and(0.94±0.08) mm for P1 point,(0.70±0.02),(0.46±0.01) and(0.60±0.03) mm for P5 point.Statistically significant difference was found between these two immobilization methods(P<0.01).The potential dose error due to position inaccuracy was within 1.2% at dose points of interest(2 cm away from the source) for patients with immobilization mask.CONCLUSIONS:The application of the custom-made nasopharyngeal applicator with the immobilization mask can significantly improve the dosimetric accuracy of HDR intracavitary brachytherapy.The method described here is universal and can be conveniently implemented in clinic.