Instituto de Física, Universidad Nacional Autónoma de México, Ciudad de México, Mexico.
Unidad de Radioterapia, Hospital Médica Sur, Ciudad de México, Mexico.
Med Phys. 2021 Nov;48(11):6567-6577. doi: 10.1002/mp.15233. Epub 2021 Sep 24.
To measure the out-of-field mean photon energy and dose imparted by the secondary radiation field generated by 6 MV and 6 MV FFF beams using TLD-300 and TLD-100 dosimeters and to use the technique to quantify the contributions from the different sources that generate out-of-field radiation.
The mean photon energy and the dose were measured using the TLD-300 glow curve properties and the TLD-100 response, respectively. The TLD-300 glow curve shape was energy-calibrated with gamma rays from Tc, F, Cs, and Co sources, and its energy dependence was quantified by a parameter obtained from the curve deconvolution. The TLD-100 signal was calibrated in absorbed dose-to-water inside the primary field. Dosimeters were placed on the linac head, and on the surface and at 4.5 cm depth in PMMA at 1-15 cm lateral distances from a 10 × 10 cm field edge at the isocenter plane. Three configurations of dosimeters around the linac were defined to identify and quantify the contributions from the different sources of out-of-field radiation.
Typical energies of head leakage were about 500 keV for both beams. The mean energy of collimator-scattered radiation was equal to or larger than 1250 keV and, for phantom-scattered radiation, mean photon energies were 400 keV for the 6 MV and 300 keV for the 6 MV FFF beam. Relative uncertainties to determine mean photon energy were better than 15% for energies below 700 keV, and 40% above 1000 keV. The technique lost its sensitivity to the incident photon energy above 1250 keV. On the phantom surface and at 1-15 cm from the field edge, 80%-90% of out-of-field dose came from scattering in the secondary collimator. At 4.5 cm deep in the phantom and 1-5 cm from the field edge, 50%-60% of the out-of-field dose originated in the phantom. At the points of measurement, the head leakage imparted less than 0.1% of the dose at the isocenter. The 6 MV FFF beam imparted 8-36% less out-of-field dose than the 6 MV beam. These energy results are consistent with general Monte Carlo simulation predictions and show excellent agreement with simulations for a similar linac. The measured out-of-field doses showed good agreement with independent evaluations.
The out-of-field mean photon energy and dose imparted by the secondary radiation field were quantified by the applied TLD-300/TLD-100 method. The main sources of out-of-field dose were identified and quantified using three configurations of dosimeters around the linac. This technique could be of value to validate Monte Carlo simulations where the linac head design, configuration, or material composition are unavailable.
使用 TLD-300 和 TLD-100 剂量计测量 6 MV 和 6 MVFFF 射束产生的次级辐射场的离轴平均光子能量和剂量，并使用该技术量化产生离轴辐射的不同源的贡献。
使用 TLD-300 发光曲线特性和 TLD-100 响应分别测量平均光子能量和剂量。TLD-300 发光曲线形状用来自 Tc、F、Cs 和 Co 源的伽马射线进行能量校准，并通过从曲线反卷积中获得的参数对其能量依赖性进行量化。TLD-100 信号在水的初级场中的吸收剂量校准。剂量计放置在直线加速器头、PMMA 表面和 4.5cm 深处，在等中心平面上从 10×10cm 射野边缘的 1-15cm 处。定义了三种剂量计配置，以识别和量化离轴辐射的不同源的贡献。
两种射束的典型头泄漏能量约为 500keV。准直器散射辐射的平均能量等于或大于 1250keV，对于体模散射辐射，6 MV 的平均光子能量为 400keV，6 MVFFF 射束的平均光子能量为 300keV。对于低于 700keV 的能量，确定平均光子能量的相对不确定度优于 15%，而对于高于 1000keV 的能量，相对不确定度为 40%。该技术对入射光子能量高于 1250keV 失去了灵敏度。在体模表面和离射野边缘 1-15cm 处，80%-90%的离轴剂量来自次级准直器的散射。在体模深处 4.5cm 和离射野边缘 1-5cm 处，50%-60%的离轴剂量来自体模。在测量点，头泄漏在等中心处产生的剂量小于 0.1%。6 MVFFF 射束比 6 MV 射束产生的离轴剂量少 8-36%。这些能量结果与一般蒙特卡罗模拟预测一致，并与类似直线加速器的模拟结果非常吻合。测量的离轴剂量与独立评估结果吻合良好。
应用 TLD-300/TLD-100 方法量化了次级辐射场的离轴平均光子能量和剂量。使用直线加速器周围的三种剂量计配置，确定并量化了离轴剂量的主要来源。该技术可用于验证直线加速器头设计、配置或材料组成不可用时的蒙特卡罗模拟。
