Chiu-Tsao S T, Anderson L L, O'Brien K, Sanna R
Department of Radiation Oncology, Henry Ford Hospital, Detroit, MI 48202.
Med Phys. 1990 Sep-Oct;17(5):815-25. doi: 10.1118/1.596586.
Dose rates in water have been determined for the two types of 125I seed currently used in brachytherapy. The need for such determinations became evident when water/air ratios measured with a silicon diode were found to be lower than expected. Extensive measurements using lithium fluoride thermoluminescent dosimeters (TLD's) have been performed in a solid water phantom, at distances from 0.1 to 10 cm from the seed center and at angular increments of 10 degrees, 15 degrees, or 30 degrees within a plane through the seed axis. Dose calibration of the TLD's was accomplished by irradiation in air with 125I seeds of the same type and of strengths traceable to a calibration at the National Institute of Standards and Technology (NIST). Relative calibration of TLD's was monitored by irradiation, in an oven-type x-ray machine, of control dosimeters simultaneously and all dosimeters intercurrently with the 125I irradiations. Values obtained for the dose rate constant, i.e., dose rate per unit air-kerma strength at 1 cm on the transverse axis, were 0.853 and 0.932 cGy h-1 U-1 (1.08 and 1.18 cGy h-1 mCi-1) for the 6711 and 6702 seeds, respectively. Measured data were supplemented with Monte Carlo-calculated relative dose rate data generated using the MORSE code. These calculations used 100 energy groups from 10 to 35.4 keV and involved energy collection bins ranging from 0.025 to 1.2 cm on an edge. Normalized at 1 cm, transverse axis calculated data are not significantly different from measured data (ours or cited literature) at distances either less than 2.5 or greater than 8 cm. Normalized at different distances along the transverse axis, our off-axis calculated and measured distributions agree closely at all angles but differ from literature measured distributions at small (less than or equal to 1 cm) distances and, for small angles, increasingly at larger distances (greater than or equal to 5 cm).
已测定了目前近距离放射治疗中使用的两种类型的¹²⁵I种子在水中的剂量率。当发现用硅二极管测量的水/空气比低于预期时，进行此类测定的必要性变得明显。使用氟化锂热释光剂量计（TLD）在固体水模体中进行了广泛测量，测量距离种子中心0.1至10厘米，在通过种子轴的平面内角度增量为10度、15度或30度。TLD的剂量校准是通过在空气中用相同类型且强度可追溯到美国国家标准与技术研究院（NIST）校准的¹²⁵I种子进行照射来完成的。TLD的相对校准通过在烤箱型X射线机中同时照射对照剂量计以及在¹²⁵I照射期间同时照射所有剂量计来监测。对于6711和6702种子，在横轴1厘米处获得的剂量率常数（即每单位空气比释动能强度的剂量率）值分别为0.853和0.932 cGy h⁻¹ U⁻¹（1.08和1.18 cGy h⁻¹ mCi⁻¹）。测量数据用使用MORSE代码生成的蒙特卡罗计算的相对剂量率数据进行了补充。这些计算使用了10到35.4 keV的100个能组，并且涉及边缘上从0.025到1.2厘米的能量收集区间。在1厘米处归一化后，横轴计算数据在小于2.5厘米或大于8厘米的距离处与测量数据（我们的或引用文献中的）没有显著差异。在横轴不同距离处归一化后，我们的离轴计算和测量分布在所有角度都非常吻合，但在小（小于或等于1厘米）距离处与文献测量分布不同，并且对于小角度，在较大（大于或等于5厘米）距离处差异越来越大。
