Monroe James I, Williamson Jeffrey F
Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St Louis, Missouri 63110, USA.
Med Phys. 2002 Apr;29(4):609-21. doi: 10.1118/1.1460876.
A dosimetric study of a 103Pd seed for permanent interstitial brachytherapy, the Theragenics Corporation Model 200 (TheraSeed), has been undertaken utilizing Monte Carlo photon-transport (MCPT) simulations. All dosimetric quantities recommended by the American Association of Physicists in Medicine (AAPM) Task Group 43 (TG-43) [Med. Phys. 22, 209-234 (1995)] report have been calculated. This source contains graphite pellets coated with palladium metal, within which the radioactive 103Pd is distributed. Due to the significant influence of this metal coating thickness on the dose distribution, two coating thicknesses, 2.2 microm (light seed, representing currently available seeds) and 10.5 microm (heavy seed, representing reactor-produced seeds available before 1994), were analyzed. Quantities determined are the following: dose rate constant, radial dose function, anisotropy function, anisotropy factor, anisotropy constant, and "along and away" dose tables. The National Institute of Standards and Technology (NIST) Wide Angle Free Air Chamber (WAFAC) standard for air-kerma strength (SK,N99) was simulated, allowing a comparison to measured dosimetry data normalized to SKN99. The calculated dose-rate constants are 0.691 (light seed) and 0.694 cGy h(-1) U(-1) (heavy seed), where 1 U= 1 microGy x m2 x h(-1), in contrast to the recommended TG-43 value of 0.74 cGy h(-1) U(-1) and the value, 0.665 cGy h(-1) U(-1), recommended by AAPM report 69 [Med. Phys. 27, 634-642 (2000)]. Anisotropy constants (1/r2 weighted average, r > or = 1 cm) are 0.862 and 0.884 for the light seed and heavy seed, respectively. A generalization of the AAPM formalism [Med. Phys. 27, 634-642] for evaluating the time-dependent ratio of an administered-to-prescribed dose is presented. The findings of this study, in combination with 5% corrections applied to WAFAC measurements performed in 1999, imply that changes in the AAPM's recommended ratios as large as 6%, are indicated.
利用蒙特卡罗光子传输（MCPT）模拟对用于永久性组织间近距离治疗的103Pd种子源（Theragenics公司的200型，TheraSeed）进行了剂量学研究。计算了美国医学物理学家协会（AAPM）任务组43（TG - 43）[《医学物理》22，209 - 234（1995）]报告中推荐的所有剂量学量。该源包含涂有钯金属的石墨小球，放射性103Pd分布在其中。由于这种金属涂层厚度对剂量分布有显著影响，分析了两种涂层厚度，2.2微米（轻种子，代表目前可用的种子）和10.5微米（重种子，代表1994年以前可用的反应堆生产的种子）。确定的量如下：剂量率常数、径向剂量函数、各向异性函数、各向异性因子、各向异性常数以及“沿向和背向”剂量表。模拟了美国国家标准与技术研究院（NIST）广角自由空气电离室（WAFAC）空气比释动能强度（SK，N99）标准，以便与归一化为SKN99的测量剂量学数据进行比较。计算得到的剂量率常数分别为0.691（轻种子）和0.694 cGy h(-1) U(-1)（重种子），其中1 U = 1微戈瑞×平方米×小时(-1)，与之相比，TG - 43推荐值为0.74 cGy h(-1) U(-1)，AAPM报告69[《医学物理》27，634 - 642（2000）]推荐值为0.665 cGy h(-1) U(-1)。轻种子和重种子的各向异性常数（1/r2加权平均值，r≥1厘米）分别为0.862和0.884。提出了一种对AAPM形式主义[《医学物理》27，634 - 642]的推广，用于评估给药剂量与规定剂量的时间依赖性比值。本研究结果与1999年对WAFAC测量进行的5%校正相结合，表明AAPM推荐比值的变化高达6%。
