The relationship of clinical factors and radiation exposure rates from iodine-131 treated thyroid carcinoma patients

Iodine-131 (I-131) was administered orally 58 times over a 31-month period to patients with thyroid carcinoma. Federal regulations (10 CFR 35) require that a patient remain hospitalized until total body activity is less than 30 mCi. This mandated a hospitalization of more than 48 hours in 10 of 58 patients (17%), and greater than 72 hours in 1 of 58 patients (2%). During each administration of I-131 exposure rates were periodically measured throughout the hospitalization at the surface of the neck, 1 meter from the neck surface, the surface of the abdomen, 1 meter from the anterior abdominal surface, and 1 meter from the lateral abdominal surface. Semi-log regression curves were generated for exposure rate versus time for various parameters, including tumor histology, extent of surgery, gender, age, TSH, site of measurement, and administered activity. The endpoint evaluated was the regressed time to reduce the exposure rate to 10% of the initial value. None of the clinical characteristics tested revealed any differences in decay times. The measurements taken 1 meter anterior to the stomach and neck surface were found to correlate best with decay time. We conclude that exposure rate assessments for the purpose of hospital discharge should be made at 1 meter anterior to the stomach or neck, and that individual patient measurements are necessary because none of the clinical parameters were predictive of I-131 elimination.