Findings of I-131 SPECT/CT, ¹⁸F-FDG, and ⁶⁸Ga-FAPI-04 PET/CT Imaging in a Patient Treated with Radioiodine Therapy for Metastatic Papillary Thyroid Carcinoma

Abstract

A 50-year-old man undergone total thyroidectomy and histopathology revealed papillary thyroid carcinoma with a tumor size of 4.5 cm. The patient was referred to a nuclear medicine clinic for radioiodine therapy. Since the thyroglobulin level before the treatment was 495 ug/L, low-dose (185 MBq) I-131 scan was performed. In addition to multiple liver metastases, bone metastases were detected in the sacrum and right 7^th rib in I-131 whole body scanning and single photon emission computed tomography/computed tomography (CT) imaging at the time of initial staging. We present a case of multiple metastatic papillary thyroid carcinoma whose radioiodine treatment response and clinical outcome were evaluated with ¹⁸F-fluorodeoxyglucose and Gallium-68 FAPI-04 positron emission tomography/CT.

Figure 1.

Anterior (a) and posterior (b) view of whole-body scan (WBS) with 185 MBq (5 mCi) I-131 were performed due to elevated stimulated thyroglobulin (Tg) (higher than 495 ug/L, anti Tg: 1.7 IU/mL) and serum liver enzymes (alanine transaminase: 112 U/L and aspartate transaminase: 70 U/L). Remnant tissue uptake was observed in the thyroid bed (arrow), and four metastatic foci in the liver (dashed arrow), right half of the sacrum (arrowhead), and right anterior 7^th rib of the thoracic skeleton (black arrowhead) were also observed. 9250 MBq (250 mCi) radioiodine therapy was applied. Post-therapy WBS and single photon emission computed tomography/computed tomography (CT) axial fusion images showed intense iodine accumulation in the metastatic lesions as described on low-dose WBS (c, d, e).

Figure 2.

¹⁸F-fluorodeoxyglucose positron emission tomography (PET)/CT was performed 8 months after the radioiodine therapy to evaluate response and determine if there are other foci with ¹⁸F-FDG positivity. Images of maximum-intensity projection (a) showed only one hypermetabolic metastatic mass-like lesion in the left lobe medial segment of the liver [maximum standardized uptake value (SUV_max): 6.5] (arrow), anterior part of the right 7_th rib (dashed arrow), and lytic destructive hypermetabolic metastatic bone lesion with central necrosis (SUV_max: 4.0) in the right half of the sacrum (d) without additional lesions to the I-131 scan. Ten months after the first radioiodine therapy, the second therapy with 9250 MBq (250 mCi) was applied and radioiodine avid known metastasis was visualized in post-therapy I-131 scan. Four months after the second radioiodine therapy, the second ¹⁸F-FDG PET/CT scan showed complete metabolic response in the liver (e) and partial response was observed in bone lesions (f, g). Serum Tg level decreased to 168 μg/L and liver enzymes returned to normal.

Figure 3.

Eight months after 2^nd radioiodine therapy, a third dose with 9250 MBq (250 mCi) I-131 was given (2750 MBq achieved) and multiple liver (a, b, c, d) and bone metastases were still iodine avid (e, f).

Figure 4.

¹⁸F-F-DG PET/CT was repeated 3 months after the third radioiodine therapy (a), and mild ¹⁸F-FDG uptake was detected in bone lesions (d, e) with continuing ¹⁸F-FDG negativity in the liver (c). In the same week, Gallium-68 (⁶⁸Ga)-FAPI-04 PET/CT imaging was performed (b) and moderate ⁶⁸Ga-FAPI-04 uptake was detected in hepatic metastases, while ¹⁸F-FDG was negative (f). Also, mild peripheral ⁶⁸Ga-FAPI-04 uptakes were seen in bone metastases (g, h) like as ¹⁸F-FDG. Four months later last ¹⁸F-FDG scan was performed and stable disease was detected with no signs of progression as to the previous scan. The last viewed Tg level decreased to 26.5 μg/L.

Distant metastatic disease at the time of diagnosis is rarely encountered in differentiated thyroid carcinoma (DTC) and usually occurs in the lungs and skeleton (1,2). Liver metastasis are seen even rarer (<1%). Radioiodine therapy is a valuable radionuclide in practice for treating DTC when they concentrate iodine (3,4). PET/CT serves a limited contribution to the diagnosis, but it has a significant impact in the management of DTC, especially at increased Tg levels (5,6). Response assessment can be investigated by PET/CT for an option to I-131 scan (7). As shown in the current case, ⁶⁸Ga- FAPI-04 may also take a place and add benefit in the restaging of metastatic DTC (8).