Ambient Radiation Dose Levels From Differentiated Thyroid Cancer Patients With ¹³¹I Treatment

ObjectiveIn this study,by monitoring the in vivo radioactivity of ¹³¹I treated patients with differentiated thyroid cancer（DTC）at different times and the external dose equivalent rate at different times and distances,we analyzed the changes of activity and dose equivalent rate and the relationship between them,and estimated the cumulative dose level around the patient under unrestricted conditions.In addition,using 400 MBq as the node to compare the difference between the dose equivalent rate estimated by the point source model and the line source model and the actual measurement results,and to estimate the correction factor of the body’s own effect on the dose equivalent rate.It is expected to provide reference for evaluating the radiation risk of DTC patients to the surrounding exposed population and environment and formulating more scientific and reasonable protective guidance recommendations.MethodsThe subjects of the study are 43 DTC patients who received ¹³¹I therapy for the first time after total thyroidectomy.The dosage was 1850-3700 MBq,and the average dosage was（2405±777）MBq.The measurements of residual radioactivity in patients and dose equivalent rate at 0.3,0.5,1,2 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 taking ¹³¹I.Results1.The residual radioactivity in patients after ¹³¹I therapy changed with time as a function is:A=A₀(1.03316e^-0.0624t+0.01717).2.The variation functions of normalized dose equivalent rate of 0.3,0.5,1,2and 3 m away from the front of the body surface of patients with time are:（?）_0.3=127.2207e^-0.0548t+3.76571,（?）_0.5=91.0288e^-0.0594t+2.08476,（?）₁=30.2258e^-0.0644t+0.82467,（?）₂=9.4318e^-0.0628t+0.28413,（?）₃=4.1619e^-0.0615t+0.16797.3.The decay function of the external dose equivalent rate of the patient with distance is:（?）=1.008×（?）₁/r ^1.632.4.There was a linear positive correlation between the residual radioactivity of patients at different times after taking¹³¹I and the corresponding dose equivalent rate at 0.3,0.5,1,2 and 3 m（r=0.953,0.980,0.982,0.986 and 0.984,P<0.05）,and the linear regression equations are:（?）_0.3=0.103A+17.239,（?）_0.5=0.074A+7.244,（?）₁=0.025A+1.245,（?）₂=0.008A+0.575,（?）₃=0.003A+0.396.5.When using the point source and line source models to estimate dose equivalence rates at 0.3,0.5,1,2 and 3 m from the patients with radioactivity around400 MBq,the correction factors for the point source model are 0.25,0.41,0.49,0.65,0.70,respectively;and the correction factors for the line source model are 0.32,0.46,0.50,0.65,0.70,respectively.6.The cumulative doses per unit activity at 0.3,0.5,1,2 and 3 m from the patients during hospitalization were 2.54778,1.66129,0.52417,0.16901 and0.07896μSv·MBq^-1,respectively.7.During the three days in the hospital,the cumulative doses at 0.3,0.5,1,2and 3 m for patients with radioactivity of 1850 MBq were 4.71,3.07,0.97,0.31 and0.15 m Sv,respectively;the cumulative doses at 0.3,0.5,1,2 and 3 m for patients with radioactivity of 2960 MBq were 7.54,4.92,1.55,0.50 and 0.23 m Sv,respectively;the cumulative doses at 0.3,0.5,1,2 and 3 m for patients with radioactivity of 3700 MBq were 9.43,6.15,1.94,0.63 and 0.29 m Sv,respectively.8.The maximum radiation dose that the public may receive in two hours of traveling with the patient with radioactivity below 3700 MBq does not exceed 45.11μSv.Conclusions1.DTC patients had the fastest decline in radioactivity on the first day after taking ¹³¹I,and there was little radioactivity left in the body three days after taking¹³¹I.DTC patients with¹³¹I activity of 3700 MBq and below only need to be hospitalized for two days to reach the discharge standard.2.The normalized dose equivalent rate at different distances from the patient decays exponentially with time,and the normalized dose equivalent rate decay rate of Chinese patients was faster than that of patients from other countries.3.When the residual activities of DTC patients were 1000,700 and 400 MBq,the corresponding dose equivalent rate at 1 m from the patient were 26.2,18.7 and11.2μSv/h,respectively.4.Using the point source model and line source model to estimate the patient’s ambient dose equivalent rate will cause overestimation.Therefore,the use of formula to estimate the radiation level around the patient requires to use the correction factor of the effect of human self-absorption on the radiation.5.During the hospitalization of DTC patients with ¹³¹I activity of 3700 MBq and below,a single patient makes routine contact with the surrounding population,that is,the contact distance is greater than or equal to 1 m,and the cumulative dose of the surrounding population will not exceed 5 m Sv,and the contact distance is greater than or equal to 2 m,the cumulative dose will not exceed the public annual dose constraint limit of 1 m Sv.