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Objective To determine the radiation dose of sensitive organs under different protective methods in lung CT scanning environment, and to explore the best protective scheme of corresponding organs. Methods Annealed thermoluminescence dose elements were placed in the stomach, liver, colon, and thyroid gland of a simulated human body model. The dose effect experiment of protective methods included non-protective group, half lead apron group, and full lead apron group. The dose effect experiment of protective thickness included 0.50 mmpb full lead apron group and 0.35 mmpb full lead apron group. The same exposure conditions of lung CT scan were used in the above experiments. Results Compared with the non-protective group, the exposure dose of the stomach, liver, colon, and thyroid gland increased significantly in the half lead apron group (P < 0.05), and the exposure dose of the thyroid gland and colon decreased significantly in the full lead apron group (P < 0.05). There were no significant differences in the exposure dose of the liver, stomach, and colon in the simulated human body model between the 0.35 mmpb full lead apron group and the 0.50 mmpb full lead apron group. Conclusion For lung CT scan, the protective measure of lead apron may not reduce the exposure dose of subjects. The protective thickness of lead apron does not necessarily have a substantial influence on the exposure dose of human body.
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Objective:To study the variation in activity in patient′s body with differentiated thyroid cancer (DTC) treated with 131I and external dose level, analyze the relationship between the both, and estimate the correction factor for the dose equivalent rate for the patients with residual activity of 400 MBq. Methods:A total of 43 DTC patients who received 131I therapy for the first time after total thyroidectomy were studied. The dose was 1 850-3 700 MBq and average dose was (2 405±777) MBq. The measurements of residual activity in patient′s body and of dose equivalent rate at 0.3, 1 and 3 m in front of the patients were performed at 2, 6, 20, 22, 24, 27, 30, 44, 46, 48, 54, 68 and 72 h after administration of 131I. Results:The residual activity in patient′s body after 131I therapy varied with time as a function of A= A0 (1.033 16e -0.062 4t+ 0.017 17). It can be estimated that the effective half-life of DTC patients treated with thyroid remnant 131I ablation therapy is 12.19 h. It needs only 26.4-38.9 h to reduce the internal activity to the 400 MBq. The functions of variation with time of normalized dose equivalent rate at 0.3, 1, and 3 m away from patients were: H· 0.3=127.220 7e -0.054 8t+ 3.765 71; H· 1=30.225 8e -0.064 4t+ 0.824 67; and H· 3=4.161 9e -0.061 5t+ 0.167 97, respectively. There was a positive correlation between residual activity and dose equivalent rate at 1 m ( r=0.982, P<0.05), and the function is H· 1=0.025 A+ 1.245. When residual activities in DTC patient′s body were 1 000, 700 and 400 MBq, the corresponding dose equivalent rates at 1 m from patients were 26.2, 18.7 and 11.2 μSv/h, respectively. The correction factors for dose equivalent rate at 0.3, 1 and 3 m from patients with 400 MBq were 0.25, 0.49 and 0.70, respectively. Conclusions:DTC patients with administration of 131I activity below 3 700 MBq need only to be hospitalized for two days to reach the discharge standards. When the residual activity in DTC patient′s body drops to 400 MBq, the dose equivalent rate at 1 m is far less than 25 μSv/h. Simply using the point source formula to estimate the dose equivalent rate around the patient will result in overestimation. Therefore, the correction factor used in the estimation of radiation doses to patients by using the formula needs to be further studied so as to make the model-based estimated result more consistent with the actual situation.