Activity variation and dose level in patient′s body with differentiated thyroid cancer after 131I therapy / 中华放射医学与防护杂志

ABSTRACT

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.