Patients with an autonomously functioning thyroid nodule (ATN) may be present with various clinical, biochemical and scintigraphic features. To optimize 131I dose planning and treatment timing in these patients, relationships between dosimetric data and clinical follow-up events must be established.
Methods: We retrospectively reviewed the records of 88 patients who received 131I (intended dose of 80 Gy) for an ATN, of whom 39 had evidence of extranodular activity (ENA) and 76 presented with overt thyrotoxicosis. In all of the patients, dosage calculation was monitored to estimate precisely both beta and gamma absorbed doses received by the ATN and the nodule-free lobe. The mean duration of follow-up was 75 mo (max 180) and always included biochemical thyroid tests. Finally, we compared the dosimetric profiles of four dosage schemes which had been normalized by simulation to ensure that the same absorbed dose threshold value was always delivered to the ATN.
Results: About 75% of the patients were cured at 6 mo for a mean 305 MBq administered. The absorbed doses delivered to the nodule-free lobe ranged from 12% (no ENA) to 86% (ENA) of the values delivered to the ATN, mainly in the form of beta irradiation. Life-table estimates for hypothyroidism and death were 9.6% and 22% at 75 mo, respectively. Hypothyroidism mainly developed in patients with nonsuppressed TSH levels but regardless of ENA, which often accounted for multifocal disease.
Conclusion: We suggest that fixed doses bordering on 370 MBq are advizable in younger individuals and in patients with mild thyrotoxocosis, while 555 MBq-740 MBq can be administered in other patients and that ENA indicates multifocal autonomy in patients with toxic ATN and is a further indication for radioiodine treatment which should be begun as soon as possible to avoid the development of cardiac complications.