Objective: To study residual radioactivity in WDTC patients after receiving high dose I-131 therapy and proportion of patients with residual radioactivity regarding discharged recommendation at different time points. Factors that may correlate to residual radioactivity were also evaluated.

Materials and Methods: Prospective cohort study in 170 thyroid cancer patients treated with I-131 3.7 to 7.4 GBq (100 to 200 mCi). Data were obtained from medical records, interview, and residual radioactivity measured at 24, 48, and 72 hours after receiving I-131. Difference of factors and between subject groups with residual radioactivity within and that exceed the discharge criteria (30 mCi) were evaluated. A p-value less than 0.05 was considered statistically significant.

Results: The median residual radioactivity at 24, 48, and 72 hours were 26.63 mCi (range 10.97 to 117.87), 7.89 mCi (range 1.27 to 76.27), and 3.27 mCi (range 0.25 to 55.47), respectively. Proportions of patients with residual radioactivity regarding discharge recommendation were 59.40%, 92.40%, and, 95.70% at 24, 48, and 72 hours, respectively. Factors correlated with residual radioactivity were age (p<0.001), serum creatinine level (p=0.042), dose received (p=0.005), and 24-hour I-131% uptake (p=0.015), while gender, BMI, fluid intake, and frequency of radiation excretion via urine and stool had no such significant effect.

Conclusion: A very high proportion of patients had residual radioactivity that met the discharge recommendation level at 48 hours after I-131 therapy. This time point may be adjusted for discharge planning. Elderly patients, higher administered dose of I-131, those with high creatinine level, or high 24-hour I-131% uptake should be advised of the risk of residual radioactivity to the public.

Keywords: Residual radioactivity, Well differentiated thyroid cancer, I-131 therapy, Patient discharge