Evaluation of Mid-Plane Dose by Flat Panel Transit Dosimetry Method and by EBT2 Film Incident Dosimetry

Purpose: The aim of this study is to validate an easily applicable flat panel dosimetry method based on the back projection approach and to compare this method with the incident dosimetry by EBT2 film method for mid-plane dose calculations. Methods: The dosimetric characteristics of the flat panel were determined for 6 MV photon energy. Then, the methodology to calculate the dose on the central axis of the photon beam was described. While, the flat panel dosimetry method was validated with phantom measurements using an ionization chamber. Once the method was validated, in vivo measurements of ten prostate patients treated with 6 MV photon energy 3D conformal plans were also performed both with the flat panel and the EBT2 films. Results: The phantom measurements revealed a mean dispersion of 1.7% between flat panel and ionization chamber doses and 2.2% between flat panel and EBT2 film doses. While, the in vivo measurements in prostate patients revealed a mean dispersion of 0.8% between flat panel doses and treatment planning calculated doses and 1% between flat panel and EBT2 film doses. Conclusions: The presented flat panel dosimetry method is accurate, easily applicable to all types of flat panels without the use of any sophisticated software and is not time consuming.

辐射自显影胶片EBT2在单次曝光与多次曝光条件下的剂量学研究

目的:研究辐射自显影胶片EBT2在全身放射治疗的剂量测量中单次辐照与多次累积辐照后的剂量学稳定性,从而判断是否可用单张胶片记录多分次治疗中的累积剂量。方法:对3张胶片分别给予200 cGy、400 cGy、800 cGy的辐照,分别在照射后10 h^96 h内进行多次测量,观察黑化时间对胶片剂量的影响;对3张胶片分别给予100 cGy、150 cGy、200 cGy的辐照,每天重复扫描5次共扫描4 d,并与第1次扫描结果相比较,分析重复扫描对胶片剂量的影响;将EBT2胶片分为2组,一组每日分别给予100 cGy、150 cGy、200 cGy的剂量照射,累计重复4 d;另一组分别一次性给予200 cGy、300 cGy、400 cGy、450 cGy、600 cGy、800 cGy,比较接受相同剂量不同照射方式下的胶片剂量。结果:辐照4 d后的胶片剂量相比于辐照24 h后的剂量要高2%~3%;累积照射4 d后的胶片剂量相比于单次接受同等剂量要低1.5%~2.5%。同时也发现扫描仪的灯光对胶片剂量并不产生明显影响。结论:辐射自显影胶片EBT2可以用于多次照射后累积剂量的测量。

放疗计划系统中空腔边缘剂量计算准确性研究

目的 研究TPS中CCC、AAA算法在空腔边缘剂量计算的准确性。方法 采用EGSnrc软件包的子程序BEAMnrc用对美国瓦里安Trilogy直线加速器进行了蒙特卡罗模拟,并使用IBA蓝水箱三维扫描系统验证了该Monte Carlo算法的准确性,证实蒙特卡罗算法的可靠性。设计一带有不同大小空腔的等效水模体,分别用CCC、AAA算法计算不同大小空腔的等效水体模的中心轴深度剂量分布和侧向剂量profile。深度方向计算结果与蒙特卡罗算法进行比较,侧向剂量profile计算结果与EBT2胶片测量结果进行比较。结果 在空腔边缘CCC、AAA算法都高估了剂量。CCC计算精确性优于AAA,但误差依然存在。这种误差主要与计算网格、射野大小、能量、空腔大小、射野数目有关。结论 TPS中的CCC和AAA剂量计算时还应充分考虑到空腔边缘的电子不平衡。