Nath R, Yue N, Shahnazi K, Bongiorni P J
Yale University School of Medicine, Department of Therapeutic Radiology, New Haven, Connecticut 06520-8040, USA.
Med Phys. 2000 Apr;27(4):655-8. doi: 10.1118/1.598925.
Recent developments in the past two years require a significant change in the dosimetry of 103Pd brachytherapy sources (Theraseed model 200, manufactured by Theragenics Corp., Atlanta, GA). Since their introduction in 1987, the air kerma strength of 103Pd sources for interstitial brachytherapy has been determined using a system of apparent activity measurement based upon the measurement of photon fluence at a reference distance along the transverse axis of the source free in air, using a NaI (T1) scintillation detector at the manufacturer's facilities. This detection system has been calibrated against a National Institute of Standards and Technology (NIST)-traceable activity standard of a 109Cd source. This system produced a highly consistent standard (within +/-2%) for over 12 years, with the exception of the last 109Cd source change in September 1997, which resulted in a change of 9% from the original 1987 standard. The second major development affecting 103Pd dosimetry is that on 13 January 1999 a primary national standard for the air kerma strength of 103Pd seeds was developed by NIST. This primary standard is based upon an absolute measurement of air kerma rate free in air at a reference distance from the source along its transverse axis using a wide angle free air chamber (WAFAC). In order to implement this new standard for the calibration of source strength in clinical dosimetry for interstitial implants, it is necessary to measure the dose-rate constant for the 103Pd seeds using a calibration of source strength based on the NIST 99 standard. In this work, a measurement of the dose-rate constant using lithium fluoride (LiF) thermoluminescent dosimeters (TLDs) in a water equivalent solid phantom is reported. The measured value of this constant is 0.65 +/- 0.05 cGy h(-1) U(-1), where the unit air kerma strength is 1 U = 1 cGy h(-1) cm2 = 1 microGy h(-1) m2, and is directly traceable to the NIST 99 standard. The implementation of the NIST 99 standard for 103Pd should be accompanied by a simultaneous adoption of the new dose-rate constant reported here. No changes in radial dose function, anisotropy function, anisotropy factor, and geometry function are needed. However, a change in prescribed dose may be necessary to deliver the same physical dose as before.
过去两年的最新进展要求对103Pd近距离放射治疗源（Theraseed model 200，由Theragenics公司制造，佐治亚州亚特兰大）的剂量测定进行重大改变。自1987年引入以来，用于间质近距离放射治疗的103Pd源的空气比释动能强度一直是在制造商的设施中，使用NaI（Tl）闪烁探测器，通过基于在空气中沿源横轴的参考距离处测量光子注量的表观活度测量系统来确定的。该检测系统已根据国家标准与技术研究院（NIST）可溯源的109Cd源活度标准进行了校准。该系统在超过12年的时间里产生了高度一致的标准（在+/-2%以内），但1997年9月最后一次更换109Cd源除外，这导致与1987年的原始标准相比有9%的变化。影响103Pd剂量测定的第二个主要进展是，1999年1月13日，NIST制定了103Pd籽源空气比释动能强度的主要国家标准。该主要标准基于使用广角自由空气电离室（WAFAC）在空气中沿源横轴的参考距离处对自由空气中空气比释动能率的绝对测量。为了在间质植入临床剂量测定中实施用于源强度校准的这一新标准，有必要使用基于NIST 99标准的源强度校准来测量103Pd籽源的剂量率常数。在这项工作中，报告了在水等效固体模体中使用氟化锂（LiF）热释光剂量计（TLD）对剂量率常数的测量。该常数的测量值为0.65 +/- 0.05 cGy h(-1) U(-1)，其中空气比释动能强度的单位1 U = 1 cGy h(-1) cm2 = 1 microGy h(-1) m2，并且可直接溯源到NIST 99标准。对103Pd实施NIST 99标准应同时采用此处报告的新剂量率常数。径向剂量函数、各向异性函数、各向异性因子和几何函数无需改变。然而，可能需要改变规定剂量以提供与以前相同的物理剂量。
