Background: The management of patients with differentiated thyroid cancer (DTC) who have elevated serum thyroglobulin (Tg) levels and negative (131)I or (123)I scans is problematic, and the decision regarding whether or not to administer (131)I therapy (a "blind" therapy) is also problematic. While (124)I positron emission tomography (PET) imaging has been shown to detect more foci of residual thyroid tissue and/or metastases secondary to DTC than planar (131)I images, the utility of a negative (124)I PET scan in deciding whether or not to consider performing blind (131)I therapy is unknown. The objective of this study was to determine whether a negative (124)I pretherapy PET scan in patients with elevated serum Tg levels and negative (131)I or (123)I scans predicts a negative (131)I posttherapy scan.
Methods: Several prospective studies have been performed to compare the radiopharmacokinetics of (124)I PET versus (131)I planar imaging in patients who 1) had histologically proven DTC, 2) were suspected to have metastatic DTC (e.g., elevated Tg, positive recent fine-needle aspiration cytology, suspicious enlarging mass), and 3) had (131)I planar and (124)I PET imaging performed. Using these criteria, we retrospectively identified patients who had an elevated Tg, a negative diagnostic (131)I/(123)I scan, a negative diagnostic (124)I PET scan, therapy with (131)I, a post-therapy (131)I scan, and a prior (131)I therapy with a subsequent positive post-(131)I therapy scan. For each scan, two readers categorized every focus of (131)I and (124)I uptake as positive for thyroid tissue/metastases or physiological.
Results: Twelve patients met the above criteria. Ten of these 12 patients (83%) had positive foci on (131)I posttherapy scan.
Conclusion: In our selected patient population, (131)I posttherapy scans are frequently positive in patients with elevated serum Tg levels, a negative diagnostic (131)I or (123)I scan, and a negative (124)I PET scan. Thus, for a patient with elevated serum Tg level, negative diagnostic (131)I planar scan, and a prior post-(131)I therapy scan that was positive, a negative (124)I PET scan will have a low predictive value for a negative post-(131)I therapy scan and should not be used to exclude the option of blind (131)I therapy.