Diagnostic Performance of 18F-FDG PET/CT in Papillary Thyroid Carcinoma with Negative 131I-WBS at first Postablation, Negative Tg and Progressively Increased TgAb Level

Abstract

Differentiated thyroid cancer (DTC) patients with negative serum thyroglobulin (Tg), negative ¹³¹I whole-body scintigraphy (¹³¹I-WBS) at first post-ablation and progressively increased TgAb level are a relatively rare entity in the follow-up after total thyroidectomy and radioactive iodine therapy. The value of ¹⁸F-FDG PET/CT in detecting the recurrence of disease in these patients has only been reported in a small case series. The goal of this study was to investigate the diagnostic accuracy of ¹⁸F-FDG PET/CT in detecting recurrent disease in these specific PTC patients and to identify risk factors for patients with positive ¹⁸F-FDG PET/CT results. Eighty-two PTC patients who had ¹⁸F-FDG PET/CT scans with negative Tg, negative ¹³¹I-WBS at first post-ablation and progressively increased TgAb levels were included. We found that the sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of ¹⁸F-FDG PET/CT in this patient group were determined as 84%, 72%, 92%, 57% and 82%, respectively. ¹⁸F-FDG PET/CT scan had a good diagnostic performance and should be performed routinely in PTC patients with negative Tg, negative ¹³¹I-WBS at first postablation and progressively increased TgAb level, especially when span for progressively increased TgAb level ≥ 3 years and/or progressively increased TgAb value up to 150 IU/mL.

Figure 1

A true-positive lesion on the left neck region was detected by ¹⁸F-FDG PET/CT. A 35-year old woman underwent total thyroidectomy with central lymph node dissection on the right neck region and radioiodine ablation for remnant PTC and thyroid tissue. ¹³¹I-WBS obtained 5 days after an oral administration of 3.7 GBq of ¹³¹I showed negative finding. Six months after ablation, the patient had negative Tg (<0.1 ng/mL) but abnormal TgAb of 108 IU/mL at TSH suppression status. Subsequently, during the follow-up 3.5 years later, TgAb level progressively increased from 108 IU/mL to 623 IU/mL. ¹⁸F-FDG PET/CT revealed ¹⁸F-FDG-avid nodal lesion with SUV_max of 4.7 in the left neck (a,b and c, crossing line). Surgical pathology confirmed the metastatic nodal lesion from PTC after left neck dissection. The patient had markedly decreased TgAb level afterwards.

Figure 2

A false-positive lesion on the left neck region was revealed by ¹⁸F-FDG PET/CT. A 62-year-old woman underwent total thyroidectomy with radical left neck dissection for PTC followed by radioiodine therapy with 3.7 GBq of ¹³¹I. Three days later, post-therapy ¹³¹I WBS was performed and showed negative results. Six months after ablation. The serum Tg level was 0.18 ng/mL and TgAb level was 46 IU/mL at TSH suppression status. TgAb level was stable for 4.2 yr after ¹³¹I therapy. But subsequently, TgAb gradually increased at TSH suppression status. TgAb level elevated from 51 IU/mL to 137 IU/mL in the next follow-up of 2.2 years. ¹⁸F-FDG PET/CT demonstrated increased foci radiotracer uptake in the left submandibular region (a,b, crossing line), which was localized by CT image to the left submandibular lymph nodes with SUV_max of 2.6 (c, crossing line). However, infectious lymph node was diagnosed on histopathology after surgery.