Radiological protection guidance for radioactive patients--new data for therapeutic 131I.

Patients leaving hospital after 131I 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 131I was measured in 77 thyrotoxic patients over a period of 1-50 days after a first therapeutic administration of the radionuclide. Mean 131I 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 131I therapy, there was evidence that 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 131I for treatment of thyrotoxicosis, thus obviating the need for serial measurements in every individual patient.