Williamson J F
Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110.
Med Phys. 1991 Jul-Aug;18(4):776-86. doi: 10.1118/1.596631.
Recent theoretical and experimental work indicates that currently accepted 125I dosimetry data may overestimate dose in water at 1 cm by 10%-24%. Among the most comprehensive measurements are those of the NCI-sponsored brachytherapy contract participants. Absolute dose rates in water calculated by the Monte Carlo method have been compared with the NCI dose measurements about 125I and 192Ir seeds embedded in solid-water phantoms. The photon transport code allows realistic geometric simulation of the complex internal seed structure, the National Institute of Standards and Technology air-kerma strength standardization geometry, and the dose measurement setup. When the appropriate measurement medium and geometry are assumed, agreement between theory and measurement is excellent, within 3% at 1 cm and averaging 3% at larger distances. However, the data do not support the water equivalence of solid water at 125I energies indicating that solid-water measurements underestimate 125I specific dose-rate constants in water by 4.3%. Because of its higher ratio of absorption to scatter, 125I dose distributions measured in solid water are less penetrating (by 35% at 10 cm) than those measured in liquid water. For model 6711, model 6702, and steel-clad 192Ir seeds, Monte Carlo calculations yielded specific dose-rate constants (assuming liquid water medium) of 0.877, 0.932, and 1.122 cGy cm2 h-1 per unit air-kerma strength, respectively. For 125I, currently accepted values are 18% and 11% larger for the two seed models.
近期的理论和实验研究表明，目前公认的¹²⁵I剂量测定数据可能会高估水中1厘米处的剂量10% - 24%。在最全面的测量中，有美国国立癌症研究所（NCI）赞助的近距离放射治疗合同参与者所做的测量。通过蒙特卡罗方法计算的水中绝对剂量率已与NCI对嵌入固体水模体中的¹²⁵I和¹⁹²Ir种子的剂量测量结果进行了比较。光子输运代码允许对复杂的内部种子结构、国家标准与技术研究院空气比释动能强度标准化几何结构以及剂量测量设置进行逼真的几何模拟。当假设采用合适的测量介质和几何结构时，理论与测量结果之间的一致性非常好，在1厘米处误差在3%以内，在较大距离处平均误差为3%。然而，这些数据并不支持在¹²⁵I能量下固体水与水等效，这表明固体水测量结果低估了水中¹²⁵I的特定剂量率常数4.3%。由于¹²⁵I的吸收与散射比值较高，在固体水中测量的¹²⁵I剂量分布的穿透性（在10厘米处低35%）低于在液体水中测量的剂量分布。对于6711型、6702型和钢包¹⁹²Ir种子，蒙特卡罗计算得出的特定剂量率常数（假设为液体水介质）分别为每单位空气比释动能强度0.877、0.932和1.122 cGy cm² h⁻¹。对于¹²⁵I，目前公认的两个种子模型的值分别大18%和11%。
