Impact of I-124 PET-CT in the loco-regional staging of differentiated thyroid carcinoma

Abstract

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Objectives: The prognostic value of FDG PET(-CT) in patients with differentiated thyroid cancer (DTC) has been well established. The aim of this study was to assess the value of I-124 PET(-CT) in terms of staging and impact on therapy decision-making in loco-regional neck DTC. Methods: We studied 17 patients, either pre-ablation (n=6) or in suspicion of a (loco-)regional recurrent DTC (n=11), based on a clinical/ultrasound neck tumour (n=3); elevated Tg only (n=2); clinical grounds (n=2); postoperative (n=2). In two patients, I-124 PET-CT was performed at the presence of Tg antibodies. An activity of 22-118 MBq I-124 was administered per orally, after TSH stimulation by T4 withdrawal (n=14) or rhTSH (n=3), with scans on day 1 and 4. Results were compared to Tg, US, CT, as well as PTWBS, made 1 and 4 days after 0,9-5,6 GBq I-131. Follow-up verification of results was performed with surgical findings, evolution of Tg, US and/or FDG-PET. Results: In the (pTII-IV) pre-ablation group, a general good concordance with US and the PTWBS was seen in 11 separate localisations. Compared to PTWBS, PET-CT identified two additional lesions (small remnant, central lymph node). Compared to US, 2/6 patients were upstaged due to additionally detected LN. However, this did not lead to a higher ablation dose, which was successful after 1,1 and 2,8 GBq, respectively. In the recurrence group, we found in 4/10 patients small LN's not seen on US (3x<1 cm, 1x1,1 cm). Two were centrally located, two lateral, one indicating a lateral LN dissection and one (1,1 cm) treated with radioiodine. In an additional two patients, metastases were found in lung (I-124-negative) or mediastinum (I-124-positive). Discrepancies in PET and CT were seen in 13/21 localisations (7/11 patients) with few PET+/CT- lesions (3 patients) but frequent PET-/CT+ lesions (7 patients), indicating probable significant iodine-negative disease in 4/7 and operability in limited disease in 2/7 patients. Conclusions: Our preliminary results are in agreement with previous authors that I-124 PET (CT) provides at least the same information as scintigraphy after I-131 ablation or treatment. The use of I-124 may be especially preferred in those patients in whom the therapeutic efficacy of I-131 remains doubtful.