Vaezzadeh Seyedali, Allahverdi Mahmoud, Nedaie Hasan A, Ay Mohammadreza, Shirazi Alireza, Yarahmadi Mehran
Department of Medical Physics and Biomedical Engineering, Tehran University of Medical Sciences, Tehran, Iran.
Med Dosim. 2013 Summer;38(2):176-83. doi: 10.1016/j.meddos.2012.11.001. Epub 2013 Jan 3.
The electron benefit transfer (EBT) GAFCHROMIC films possess a number of features making them appropriate for high-quality dosimetry in intensity-modulated radiation therapy (IMRT). Compensators to deliver IMRT are known to change the beam-energy spectrum as well as to produce scattered photons and to contaminate electrons; therefore, the accuracy and validity of EBT-film dosimetry in compensator-based IMRT should be investigated. Percentage-depth doses and lateral-beam profiles were measured using EBT films in perpendicular orientation with respect to 6 and 18 MV photon beam energies for: (1) different thicknesses of cerrobend slab (open, 1.0, 2.0, 4.0, and 6.0 cm), field sizes (5×5, 10×10, and 20×20 cm(2)), and measurement depths (Dmax, 5.0 and 10.0 cm); and (2) step-wedged compensator in a solid phantom. To verify results, same measurements were implemented using a 0.125 cm(3) ionization chamber in a water phantom and also in Monte Carlo simulations using the Monte Carlo N-particle radiation transport computer code. The mean energy of photons was increased due to beam hardening in comparison with open fields at both 6 and 18 MV energies. For a 20×20 cm(2) field size of a 6 MV photon beam and a 6.0 cm thick block, the surface dose decreased by about 12% and percentage-depth doses increased up to 3% at 30.0 cm depth, due to the beam-hardening effect induced by the block. In contrast, at 18 MV, the surface dose increased by about 8% and depth dose reduced by 3% at 30.0 cm depth. The penumbral widths (80% to 20%) increase with block thickness, field size, and beam energy. The EBT film results were in good agreement with the ionization chamber dose profiles and Monte Carlo N-particle radiation transport computer code simulation behind the step-wedged compensator. Also, there was a good agreement between the EBT-film and the treatment-planning results on the anthropomorphic phantom. The EBT films can be accurately used as a 2D dosimeter for dose verification and quality assurance of compensator-based C-IMRT.
电子受益转移（EBT）GAFCHROMIC 薄膜具有许多特性，使其适用于调强放射治疗（IMRT）中的高质量剂量测定。已知用于实施 IMRT 的补偿器会改变射束能谱，产生散射光子并污染电子；因此，应研究基于补偿器的 IMRT 中 EBT 薄膜剂量测定的准确性和有效性。使用 EBT 薄膜在相对于 6 和 18 MV 光子束能量垂直的方向上测量百分深度剂量和侧向射束轮廓，用于：（1）不同厚度的蜡模板（开放、1.0、2.0、4.0 和 6.0 cm）、射野尺寸（5×5、10×10 和 20×20 cm²）以及测量深度（Dmax、5.0 和 10.0 cm）；（2）固体模体中的楔形补偿器。为了验证结果，在水体模中使用 0.125 cm³ 的电离室以及使用蒙特卡罗 N 粒子辐射传输计算机代码进行蒙特卡罗模拟来进行相同的测量。与 6 和 18 MV 能量下的开放射野相比，由于射束硬化，光子的平均能量增加。对于 6 MV 光子束的 20×20 cm² 射野尺寸和 6.0 cm 厚的挡块，由于挡块引起的射束硬化效应，表面剂量降低了约 12%，在 30.0 cm 深度处百分深度剂量增加了高达 3%。相比之下，在 18 MV 时，表面剂量增加了约 8%，在 30.0 cm 深度处深度剂量降低了 3%。半值宽度（80%至 20%）随挡块厚度、射野尺寸和射束能量增加。EBT 薄膜的结果与楔形补偿器后方的电离室剂量分布以及蒙特卡罗 N 粒子辐射传输计算机代码模拟结果吻合良好。此外，EBT 薄膜与人体模体上的治疗计划结果之间也吻合良好。EBT 薄膜可准确用作二维剂量计，用于基于补偿器的适形调强放疗的剂量验证和质量保证。
