RESUMO
UNLABELLED: A dose limit-based criterion was proposed to authorize hospital discharge of thyroid carcinoma patients treated with 131I. Evaluation of accumulated doses to determine the effective half-life, the expected accumulated dose at 1 m, and the hospitalization time was performed to ensure that the dose limit was satisfied for each patient. Situations involving different dose limits and occupancy factors were analyzed. This study dealt only with external exposure; the problem of internal contamination was not considered. METHODS: Fourteen patients treated postoperatively with 131I were studied. The range of activity was 1,110-8,175 MBq. Electronic dosimeters and thermoluminescent dosimeter chips were placed on the left pectoral muscle. Dose was measured for a mean of approximately 2.5 d. The accumulated doses were plotted as a function of time and then fitted using an exponential model to obtain the parameters of total accumulated dose and effective half-life. The doses to the public and relatives at 1 m were calculated with point source approximation and several occupancy factors. RESULTS: The fit function parameters of accumulated doses in the first 36 h predicted the behavior of the total accumulated dose within a 5% error in the parameters. Estimated values of the accumulated dose 1 m from the patient were generally <5 mSv, even for an occupancy factor of 100%. For more restrictive dose constraints, hospitalization times were calculated according to different occupancy factors, as suggested in the European Commission guide. From the fit of the measured data, values of effective half-life for each patient were obtained. CONCLUSION: To apply the dose limit-based criterion, one must calculate the patient-specific parameters, as can be done using the accumulated dose. Knowledge of patient-specific parameters ensures that the patient will not expose any individual to levels greater than the dose limit. The calculated hospitalization times were less than those recommended, especially for countries with more restrictive dose limits. The type of measurements performed in this study reveals more realistic doses for the treatment of thyroid carcinoma with 131I.