Effectiveness of [(124)I]-PET/CT and [(18)F]-FDG-PET/CT for localizing recurrence in patients with differentiated thyroid carcinoma

Abstract

Although the prognosis of patients with differentiated thyroid carcinoma (DTC) is generally encouraging, a diagnostic dilemma is posed when an increasing level of serum thyroglobulin (Tg) is noted, without detection of a recurrent tumor using conventional imaging tools such as the iodine-131 whole-body scanning (the [(131)I] scan) or neck ultrasonography (US). The objective of the present study was to evaluate the diagnostic value of [(124)I]-PET/CT and [(18)F]-FDG-PET/CT in terms of accurate detection of both iodine- and non-iodine-avid recurrence, compared with that of conventional imaging such as the [(131)I] scan or neck ultrasonography (US). Between July 2009 and June 2010, we prospectively studied 19 DTC patients with elevated thyroglobulin levels but who do not show pathological lesions when conventional imaging modalities are used. All involved patients had undergone total thyroidectomy and radioiodine (RI) treatment, and who had been followed-up for a mean of 13 months (range, 6-21 months) after the last RI session. Combined [(18)F]-FDG-PET/CT and [(124)I]-PET/CT data were evaluated for detecting recurrent DTC lesions in study patients and compared with those of other radiological and/or cytological investigations. Nine of 19 patients (47.4%) showed pathological [(18)F]-FDG (5/19, 26.3%) or [(124)I]-PET (4/19, 21.1%) uptake, and were classed as true-positives. Among such patients, disease management was modified in six (66.7%) and disease was restaged in seven (77.8%). In particular, the use of the described imaging combination optimized planning of surgical resection to deal with locoregional recurrence in 21.1% (4/19) of patients, who were shown to be disease-free during follow-up after surgery. Our results indicate that combination of [(18)F]-FDG-PET/CT and [(124)I]-PET/CT affords a valuable diagnostic method that can be used to make therapeutic decisions in patients with DTC who are tumor-free on conventional imaging studies but who have high Tg levels.

Keywords: Differentiated Thyroid Carcinoma; Elevated Thyroglobulin Levels; Negative [131I] Whole-Body Scan; PET/CT; Recurrence; [124I]-PET; [18F]-FDG-PET.

Fig. 1

Diagnostic and therapeutic flow-chart of treatment performed at least 6 months after high-dose ablation.

Fig. 2

Patient 8: [¹⁸F]-FDG-PET/CT data from a patient with an elevated Tg level but who was tumor-negative on both neck US and a post-therapy [¹³¹I] scan. [¹³¹I]-negative [¹⁸F]-FDG-positive metastases are evident in the cervical lymph node (A), and the CT scan affords exact localization (B).

Fig. 3

Patient 3: [¹²⁴I]-PET/CT data from a patient with an elevated Tg level but who was tumor-negative on both neck US and a post-therapy [¹³¹I] scan. (A) The post-therapy [¹³¹I] scan did not clearly reveal the cervical lymph node metastasis evident on the [¹²⁴I]-PET scan. (B) Exact localization of the lesions of lateral lymph node metastasis, obtained by fusion of the image with that of the CT scan.

Fig. 4

Patient 11: [¹²⁴I]-PET showed that lung metastases were present; the patient had widely invasive follicular carcinoma. In this patient, no definite positive lesion was evident upon either the diagnostic or post-therapy [¹³¹I] scans, despite the presence of an elevated Tg level. The [¹²⁴I]-PET scan showed definite lung metastases (A), and [¹²⁴I]-PET/CT precisely localized the lesions (B).