Yue Ning, Nath Ravinder
Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, Connecticut 06520-8040, USA.
Med Phys. 2002 Jun;29(6):1120-9. doi: 10.1118/1.1481876.
Since the publication of the AAPM Task Group 43 report in 1995, Model 200 103Pd seed, which has been widely used in prostate seed implants and other brachytherapy procedures, has undergone some changes in its internal geometry resulting from the manufacturer's transition from lower specific activity reactor-produced 103Pd ("heavy seeds") to higher specific activity accelerator-produced radioactive material ("light seeds"). Based on previously reported theoretical calculations and measurements, the dose rate constants and the radial dose functions of the two types of seeds are nearly the same and have already been reported. In this work, the anisotropy function of the "light seed" was experimentally measured and an averaging method for the determination of the anisotropy constant from distance-dependent values of anisotropy factors is presented based upon the continuous low dose rate irradiation linear quadratic model for cell killing. The anisotropy function of Model 200 103Pd "light seeds" was measured in a Solid Water phantom using 1 X 1 x 1 mm micro LiF TLD chips at radial distances of 1, 2, 3, 4, 5, and 6 cm and at angles from 0 to 90 degrees with respect to the longitudinal axis of the seeds. At a radial distance of 1 cm, the measured anisotropy function of the 103Pd "light seed" is considerably lower than that of the 103Pd "heavy seed" reported in the TG 43 report. Our measured values at all radial distances are in excellent agreement with the results of a Monte Carlo simulation reported by Weaver, except for points along and near the seed longitudinal axis. The anisotropy constant of the 103Pd "light seed" was calculated using the linear quadratic biological model for cell killing in 30 clinical implants. For the model 200 "light seed," it has a value of 0.865. However, our biological model calculations lead us to conclude that if the anisotropy factors of an interstitial brachytherapy seed vary significantly over radial distances anisotropy constant should not be used as an approximation for anisotropy characteristics of a brachytherapy seed.
自1995年美国医学物理师协会任务组43报告发布以来，广泛应用于前列腺籽源植入及其他近距离放射治疗程序的200型103钯籽源，因其制造商从低比活度反应堆生产的103钯（“重籽源”）过渡到高比活度加速器生产的放射性材料（“轻籽源”），其内部几何结构发生了一些变化。基于先前报道的理论计算和测量结果，两种籽源的剂量率常数和径向剂量函数几乎相同，且已有报道。在这项工作中，对“轻籽源”的各向异性函数进行了实验测量，并基于细胞杀伤的连续低剂量率照射线性二次模型，提出了一种根据距离相关的各向异性因子值确定各向异性常数的平均方法。使用1×1×1毫米的微型LiF TLD芯片，在Solid Water模体中，于相对于籽源纵轴1、2、3、4、5和6厘米的径向距离以及0至90度的角度下，测量了200型103钯“轻籽源”的各向异性函数。在1厘米的径向距离处，测量得到的103钯“轻籽源”的各向异性函数明显低于TG 43报告中报道的103钯“重籽源”的各向异性函数。除了沿籽源纵轴及其附近的点外，我们在所有径向距离处的测量值与韦弗报告的蒙特卡罗模拟结果高度吻合。在30例临床植入中，使用细胞杀伤的线性二次生物学模型计算了103钯“轻籽源”的各向异性常数。对于200型“轻籽源”，其值为0.865。然而，我们的生物学模型计算使我们得出结论：如果组织间近距离放射治疗籽源的各向异性因子在径向距离上变化显著，则各向异性常数不应被用作近距离放射治疗籽源各向异性特征的近似值。
