Therapeutic 131I in outpatients: a simplified method conforming to the Code of Federal Regulations, title 10, part 35.75.

UNLABELLED: The Code of Federal Regulations, title 10, part 35.75 (10CFR35.75), provides greater latitude and flexibility in the dosing and management of outpatients treated with therapeutic 131I than did preceding regulations. Prescribing physicians should consider applying these new regulations to enhance patient convenience and lower the cost of managing appropriate outpatients. Managed care organizations and third-party payers may require that all eligible patients be treated as outpatients or that justification for hospital admission be specifically documented. To facilitate application of the code and guidelines, maximum 131I doses for patients undergoing thyroid remnant ablation, therapy for metastatic or recurrent thyroid cancer, or therapy for hyperthyroidism have been calculated and summarized in tables.

METHODS: A model was developed that calculates the maximum dose of 131I that may be dispensed to an outpatient. This model complies with 10CFR35.75. The maximum dose is calculated as a function of 5 variables: the occupancy factors for 3 periods after dose administration, the fractional uptake of 131I by residual thyroid tissue or metastasis, and the duration of constrained activity. Occupancy factor, a key new concept in the regulatory guidelines, is a physician estimate of the time that a treated patient will be near the individual with whom the patient will spend the most time after treatment. The model also considers 3 constants: the effective half-life of 131I during the preequilibrium period, and the effective half-lives of 131I in both the thyroidal component and the extrathyroidal component during the equilibrium period. Tables for maximum allowable patient 131I doses were derived on the basis of this model.

RESULTS: Through dosing charts, maximum 131I therapy doses may easily be calculated. Most outpatients undergoing thyroid remnant ablation, therapy for metastatic or recurrent thyroid cancer, or therapy for hyperthyroidism may be treated with 7400 MBq (200 mCi) 131I or more.

CONCLUSION: If the prescribing physician understands the concept of occupancy factor and how to use the dosing charts, our model facilitates application of and adherence to 10CFR35.75.