Das R K, Li Z, Perera H, Williamson J F
Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St Louis, MO 63110, USA.
Phys Med Biol. 1996 Jun;41(6):995-1006. doi: 10.1088/0031-9155/41/6/004.
Practical dosimeters in brachytherapy, such as thermoluminescent dosimeters (TLD) and diodes, are usually calibrated against low-energy megavoltage beams. To measure absolute dose rate near a brachytherapy source, it is necessary to establish the energy response of the detector relative to that of the calibration energy. The purpose of this paper is to assess the accuracy of Monte Carlo photon transport (MCPT) simulation in modelling the absolute detector response as a function of detector geometry and photon energy. We have exposed two different sizes of TLD-100 (LiF chips) and p-type silicon diode detectors to calibrated 60Co, HDR source (192Ir) and superficial x-ray beams. For the Scanditronix electron-field diode, the relative detector response, defined as the measured detector readings per measured unit of air kerma, varied from 38.46 V cGy-1 (40 kVp beam) to 6.22 V cGy-1 (60Co beam). Similarly for the large and small chips the same quantity varied from 2.08-3.02 nC cGy-1 and 0.171-0.244 nC cGy-1, respectively. Monte Carlo simulation was used to calculate the absorbed dose to the active volume of the detector per unit air kerma. If the Monte Carlo simulation is accurate, then the absolute detector response, which is defined as the measured detector reading per unit dose absorbed by the active detector volume, and is calculated by Monte Carlo simulation, should be a constant. For the diode, the absolute response is 5.86 +/- 0.15 (V cGy-1). For TLDs of size 3 x 3 x 1 mm3 the absolute response is 2.47 +/- 0.07 (nC cGy-1) and for TLDs of 1 x 1 x 1 mm3 it is 0.201 +/- 0.008 (nC cGy-1). From the above results we can conclude that the absolute response function of detectors (TLDs and diodes) is directly proportional to absorbed dose by the active volume of the detector and is independent of beam quality.
近距离放射治疗中的实用剂量仪，如热释光剂量仪（TLD）和二极管，通常是根据低能兆伏级射线束进行校准的。为了测量近距离放射治疗源附近的绝对剂量率，有必要确定探测器相对于校准能量的能量响应。本文的目的是评估蒙特卡罗光子传输（MCPT）模拟在将绝对探测器响应建模为探测器几何形状和光子能量函数时的准确性。我们将两种不同尺寸的TLD-100（氟化锂芯片）和p型硅二极管探测器暴露于校准后的60Co、高剂量率后装治疗源（192Ir）和浅表X射线束下。对于Scanditronix电子场二极管，相对探测器响应定义为每测量单位空气比释动能的测量探测器读数，其范围从38.46 V cGy-1（40 kVp射线束）到6.22 V cGy-1（60Co射线束）。同样，对于大尺寸和小尺寸芯片，该量分别从2.08 - 3.02 nC cGy-1和0.171 - 0.244 nC cGy-1变化。蒙特卡罗模拟用于计算每单位空气比释动能下探测器活性体积的吸收剂量。如果蒙特卡罗模拟准确，那么绝对探测器响应（定义为活性探测器体积每吸收单位剂量的测量探测器读数，由蒙特卡罗模拟计算得出）应该是一个常数。对于二极管，绝对响应为5.86 ± 0.15（V cGy-1）。对于尺寸为3×3×1 mm3的TLD，绝对响应为2.47 ± 0.07（nC cGy-1），对于1×1×1 mm3的TLD，绝对响应为0.201 ± 0.008（nC cGy-1）。从上述结果我们可以得出结论，探测器（TLD和二极管）的绝对响应函数与探测器活性体积的吸收剂量成正比，且与射线质无关。
