Do Negative 124 I Pretherapy Positron Emission Tomography Scans in Patients with Elevated Serum Thyroglobulin Levels Predict Negative 131 I Posttherapy Scans?

Abstract

Background:

The management of patients with differentiated thyroid cancer (DTC) who have elevated serum thyroglobulin (Tg) levels and negative ¹³¹I or ¹²³I scans is problematic, and the decision regarding whether or not to administer ¹³¹I therapy (a “blind” therapy) is also problematic. While ¹²⁴I positron emission tomography (PET) imaging has been shown to detect more foci of residual thyroid tissue and/or metastases secondary to DTC than planar ¹³¹I images, the utility of a negative ¹²⁴I PET scan in deciding whether or not to consider performing blind ¹³¹I therapy is unknown. The objective of this study was to determine whether a negative ¹²⁴I pretherapy PET scan in patients with elevated serum Tg levels and negative ¹³¹I or ¹²³I scans predicts a negative ¹³¹I posttherapy scan.

Methods:

Several prospective studies have been performed to compare the radiopharmacokinetics of ¹²⁴I PET versus ¹³¹I 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 ¹³¹I planar and ¹²⁴I PET imaging performed. Using these criteria, we retrospectively identified patients who had an elevated Tg, a negative diagnostic ¹³¹I/¹²³I scan, a negative diagnostic ¹²⁴I PET scan, therapy with ¹³¹I, a post-therapy ¹³¹I scan, and a prior ¹³¹I therapy with a subsequent positive post-¹³¹I therapy scan. For each scan, two readers categorized every focus of ¹³¹I and ¹²⁴I 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 ¹³¹I posttherapy scan.

Conclusion:

In our selected patient population, ¹³¹I posttherapy scans are frequently positive in patients with elevated serum Tg levels, a negative diagnostic ¹³¹I or ¹²³I scan, and a negative ¹²⁴I PET scan. Thus, for a patient with elevated serum Tg level, negative diagnostic ¹³¹I planar scan, and a prior post-¹³¹I therapy scan that was positive, a negative ¹²⁴I PET scan will have a low predictive value for a negative post-¹³¹I therapy scan and should not be used to exclude the option of blind ¹³¹I therapy.