Department of Medicine, Graduate School of Sapporo Medical University, Nishi 17, Minami 1 Jo, Chuo-ku, Sapporo 060-8556, Japan.
Med Phys. 2011 Jun;38(6):3069-76. doi: 10.1118/1.3590370.
The aim of this study was to develop a dose calculation method which is applicable to the interseed attenuation and the geometry lacking the equilibrium radiation scatter conditions in brachytherapy.
The dose obtained from measurement with a radiophotoluminescent glass rod dosimeter (GRD) was compared to the dose calculated with the Monte Carlo (MC) code "EGS5," using the 125I source structure detailed in by Kennedy et al. The GRDs were irradiated with 125I Oncoseed 6711 in a human head phantom. The phantom was a cylinder made of 2 mm thick PMMA with a diameter of 18 cm and length of 16 cm. Some of the GRD positions were so close to the phantom surface that the backscatter margin was less than 5 cm, insufficient for photons.
The EGS5 simulations were found to reproduce the relative dose distributions as measured with the GRDs to within 25% uncertainty in the geometry lacking the equilibrium radiation scatter conditions. The absolute value of the GRD measurement agreed with the American Association of Physicist in Medicine Task Group No 43 Updated Protocol (AAPM-TG43U1) formalism to within 3% of the reference point (r = 1 cm, theta = 90 degrees), where the TG43U1 is especially reliable because of the abundant data accumulation in composing the formalism. The factor to normalize the measured or calculated dose to the TG43U1 estimate at the reference point was evaluated to be 0.97 for the GRD measurement and 1.8 for the MC calculation, which uses the integration of the apparent activity with the time as the amount of disintegration during the irradiation. Also, F(r,theta) and g(r) estimated by this calculation method were consistent with those proposed in the TG43U1.
The results of this investigation support the validity of both the MC calculation method and GRD measurement in this study as well as the TG-43U1 formalism. Also, this calculation is applicable to interseed attenuation and the geometry lacking the equilibrium radiation scatter.
本研究旨在开发一种剂量计算方法，该方法适用于近距离放射治疗中种子衰减和缺乏平衡辐射散射条件的几何形状。
使用 Kennedy 等人详细描述的 125I 源结构，将通过放射性光致发光玻璃棒剂量计（GRD）测量获得的剂量与蒙特卡罗（MC）代码“EGS5”计算的剂量进行比较。GRD 用 125I Oncoseed 6711 在人体头部模型中进行了照射。该模型是一个由 2 毫米厚的 PMMA 制成的圆柱体，直径为 18 厘米，长度为 16 厘米。一些 GRD 位置离模型表面非常近，后散射余量小于 5 厘米，不足以用于光子。
发现 EGS5 模拟在缺乏平衡辐射散射条件的情况下，能够以 25%的不确定度再现与 GRD 测量相对剂量分布的情况。GRD 测量的绝对值与美国医学物理学家协会任务组第 43 号更新协议（AAPM-TG43U1）形式在参考点（r = 1cm，theta = 90 度）处的偏差在 3%以内，AAPM-TG43U1 特别可靠，因为在编制形式时积累了大量数据。在参考点处，将测量或计算的剂量归一化为 TG43U1 估计值的因子，对于 GRD 测量为 0.97，对于使用辐射期间的活度随时间积分的 MC 计算为 1.8。此外，该计算方法估算的 F（r，theta）和 g（r）与 TG43U1 中提出的一致。
本研究的 MC 计算方法和 GRD 测量结果以及 TG-43U1 形式的有效性得到了验证。此外，该计算方法适用于种子衰减和缺乏平衡辐射散射的情况。
