RADIOLOGICAL PROTECTION GUIDANCE FOR RADIOACTIVE PATIENTS - NEW DATA FOR THERAPEUTIC I-131

Patients leaving hospital after I-131 treatment for thyrotoxicosis face restrictions on their contact with other members of the public. These restrictions depend on the level of residual body radioactivity which for practical purposes can be taken to be almost entirely in the thyroid gland. This study provides an appropriate data base from which to draw advice to patients consistent with current radiological protection requirements in terms of the duration of these restrictions. Thyroidal retention of I-131 was measured in 77 thyrotoxic patients over a period of 1-50 days after a first therapeutic administration of the radionuclide. Mean I-131 activity in the gland (+/- S.D.) at 1 day was 56.1 +/- 11.1% of the administered dose activity and thereafter retention followed a single exponential decay pattern with a mean effective half-life (+/- S.E.M.) of 6.35 +/- 0.14 days. In patients who required further I-131 therapy, there was evidence of retention could be markedly reduced if there was virtual ablation of thyroid tissue. It is proposed that these retention data can be used to determine body radioactivity at any interval after the administration of I-131 for treatment of thyrotoxicosis, thus obviating the need for serial measurements in every individual patient.