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Value of protein-bound radioactive iodine measurements in the management of differentiated thyroid cancer treated with ¹³¹I

¹ Departments of Nuclear Medicine ² Physics ³ Clinical Oncology Thyroid Unit, Royal Marsden Hospital NHS Trust, Downs Road, Sutton, Surrey SM2 5PT, UK

Measurement of the protein-bound radioactive iodine level (PBI¹³¹) in the plasma of patients following ¹³¹I-iodide administration for thyroid cancer has been re-examined in a retrospective study of 171 patient episodes. It is shown that whereas the previously used threshold value for the measurement at 6 days does not correlate well with the 3-day whole body scan, there is good agreement between the scan and the temporal changes in PBI¹³¹ from 1–6 days: an increasing PBI¹³¹ correlates with a positive scan, and a decreasing PBI¹³¹ with a negative scan. The area under the curve (AUC) for the PBI¹³¹–time curve is related to the absorbed dose for the tumour. For a small group of 11 patients, dosimetry estimates were made from serial scans, quantified with phantoms; these absorbed doses correlated with the AUC and the 6-day PBI¹³¹. Therefore, it is suggested that these parameters may be useful in predicting absorbed radiation dose in these patients.