131I Therapy in Patients with Differentiated Thyroid Cancer: Study of External Dose Rate Attenuation Law and Individualized Patient Management.

BACKGROUND: Therapy with 131I is a well-established treatment method for post-surgical differentiated thyroid carcinoma (DTC). A fixed discharge time is generally set regardless of individual differences in residual body radioactivity (RBA). We aimed to study the RBA of each patient to find the attenuation law and to underlying factors in order to predict the time point for a safe, scientifically sound discharge plan.

METHODS: 231 DTC patients undergoing 131I treatment were all treated with 3.7 GBq (100 mCi) of 131I. RBA was estimated by measuring the external body dose rate (EDR) at a distance of one meter from the body surface between 0 to 72 hours after oral administration of 131I. Data from each patient was used to establish a time-EDR value (h-μSv/h) curve. We developed a predictive software to predict the time when a patient's dose equivalent meets the national safety standard by including six time points between 40 and 60 hours. Several factors that might affect that time were analyzed.

RESULTS: The EDR attenuation law in patients could be described with a double exponential decay model and the cutoff value was set as 23.3 μSv/h, upon which the predictive software was developed. Student's t-test showed there was no statistical difference between predicted values and the actual measured values (P＞0.05). Correlation analysis found that serum Tg, TT3, TT4, FT3, FT4, TSH, 2-hour and 24-hour iodine uptake rate of the thyroid, scores of 99mTc-pertechnetate thyroid scan, scores of 131I whole-body scan, scores of ultrasound scan and gastrointestinal residues were associated with attenuation speed. A further multiple linear regression analysis found that 24-hour iodine uptake (X1), residual thyroid grading by 131I whole-body scan (X2), blood free T3 (X3) and free T4 (X4) predominantly influenced the decline of the EDR; the regression equation was Y ̂= 2.091X_1+6.370X_2+4.529X_3+2.466X_4-8.614 (F＝44.03, P＜0.01).

CONCLUSIONS: Our study created an effective and convenient method to measure and predict the individual safety time for discharge. This could play a significant role not only in scientific hospital discharge planning, rational use of medical resource and better individualized management but in public radiation protection as well.