蒙特卡罗方法计算后装192Ir源散射校正因子研究

Development on Monte Carlo methodology with scatter correction factor of afterloading 192Ir source

目的研究一种方便、可行地推算医用后装192Ir源空气比释动能散射校正因子的方法，便于医院用指形电离室进行活度测量的QA工作的开展。方法用指形电离室测量有铅挡块和无铅挡块192Ir源空气比释动能，根据国际原子能机构（IAEA）1079号报告，计算192Ir源散射校正因子。用蒙特卡罗（MC）方法模拟测量条件，计算192Ir源散射校正因子，并与实验结果进行比较验证，同时模拟几种不同电离室型号和房间尺寸，计算并给出不同192Ir源散射校正因子。结果蒙特卡罗方法模拟192Ir源散射校正因子与实验测得的散射校正因子比较，相对误差为0．8％。利用蒙卡计算得散射校正因子推算出的源活度和用井型电离室测量推算出的源活度相差2．4％。MC模拟IAEA1079号报告中的两种球形电离室计算结果与报告中给出的结果比较，相对误差范围在0．3％～0．4％。模拟5种不同型号指形电离室，不同房间尺寸，相对误差范围在3％之内。结论用蒙卡方法模拟计算后装192Ir源散射校正因子的方法是可行的，此方法方便了医院用指形电离室进行近距离治疗QA工作的开展。

Objective To facilitate activity measurement by using the thimble ionization chamber in hospitals, to obtain air kerma scatter correction factor of medical afterloading of 192Ir source by developing an available and convenient calculation method. Methods According to International Atomic Energy Agency （IAEA） 1079 Report to calculate the scatter correction factor of 192ir source, to measure air kerma of 192Ir source with and without lead shield using thimble ionization chamber. Simulation measurement conditions were used to calculate scatter correction factor of 192Ir source and comparison was made between experimental results and literature records. At the same time, the different ionization chamber models were simulated at different room sizes to obtain scattering correction factor of 192Ir source. Results Comparison was made between the simulation scatter correction factors of 192Ir source and experiment by the shadow shield, and the relative deviation was 0.8%. The deviation of the 192Ir activity calculated according to the simulated scatter correction factor and measured by well type ionization chamber was 2.4%. By comparison between the calculated results by using two kinds of spherical ionization chamber and those ones deduced by IAEA 1079 Report, the relative deviations ranged within 0. 3%- 0.4%. Five different types of thimble ionization chamber and different room sizes were simulated and calculated by MC simulation, with the relative deviation within3%. Conclusions Monte Carlo simulation method for calculating afterloading 192ir source＇s scatter correction factor is feasible, and this method is convenient for use in the thimble chamber for brachytherapy QA work in the hospital.