[¹⁸F]FAPI-42 PET/CT in differentiated thyroid cancer: diagnostic performance, uptake values, and comparison with 2-[¹⁸F]FDG PET/CT

This study was approved by the Institutional Review Board of the Affiliated Hospital of Guilin Medical University (institutional review board number: 2022WJWZCLL-01) and was approved by the institutional review board and registered on the Chinese Clinical Trial Registry website (http://​www.​chictr.​org.​cn, number ChiCTR2200063441). All patients signed an informed consent form prior to participating, and all procedures were carried out according to the Helsinki Declaration. Patients were consecutively recruited from October 2021 to May 2022 at the Affiliated Hospital of Guilin Medical University. The inclusion criteria were as follows: (i) patients with pathologically confirmed DTC who had received thyroidectomy (followed by ¹³¹I ablation); (ii) patients with high Tg levels (suppressed Tg levels ≥ 1 ng/mL or stimulated Tg levels ≥ 10 ng/mL) and increasing Tg-Ab levels (TgAb > 115 IU/mL considered positive), irrespective of TSH levels during the follow-up. The exclusion criteria were as follows: (i) patients without DTC; (ii) patients unwilling to undergo [¹⁸F]FAPI PET/CT; (iii) patients with secondary cancer; (iv) patients who did not provide written informed consent. Data, including demographic characteristics, tumour characteristics, and clinical information, were collected from medical records.

¹⁸F was produced in situ using a GE PET-trace 800 cyclotron system (GE, US) via irradiation of ¹⁸O-H2O with 16.5-MeV protons. The radiolabelling precursors of 2-[¹⁸F]FDG were provided by Shaanxi Zhengze Biotechnology Co., Ltd. 2-[¹⁸F]FDG was synthesised in our laboratory using X5-FDG (Shaanxi Zhengze Biotechnology Co. Ltd., China) module. The radiolabelling precursors of [¹⁸F]FAPI-42 were obtained from Jiangsu Huayi Technology Co. (Jiangsu, China) with high chemical purity (> 95%). A detailed description of the radiosynthesis process (GE modules) and the quality control of the [¹⁸F]FAPI-42 has been provided in prior studies [18, 19]. Radiochemical purity was determined using radio-TLC (AR-2000, BIOSCAN, USA) and radio-HPLC (UVIS-201, Alltech, USA), which was found to be ≥ 95%.

All 2-[¹⁸F] FDG PET/CT acquisitions were conducted in accordance with the international guidelines of the European Association of Nuclear Medicine [20]. Patients were not allowed to eat any food at least 6 h before the start of FDG PET/CT (i.e. with respect to the time of injection of FDG), thereby maintaining venous blood glucose levels of < 11 mmol/L (200 mg/dL) before 2-[¹⁸F]FDG administration. However, this was not required for [¹⁸F]FAPI-42 PET/CT acquisition. After injection of [¹⁸F]FAPI-42 with an activity of 215 MBq (range, 185–303 MBq) and 2-[¹⁸F]FDG with an activity of 246 MBq (range, 166–355 MBq), whole-body images were taken from the head to the middle of the thigh at approximately 1 h. All images were obtained using the Ingenuity TF PET/CT system (Philips, Amsterdam, Holland). After CT images were obtained (CT parameters: 120 kV, 250 mAs/slice, 600-mm transaxial FOV, no gap, collimation of 64 × 0.625 mm, the pitch of 0.8, rotation time of 0.75 s, slice thickness of 1 mm, and 512 × 512 matrices), PET images were taken at the bedside at 2.5 min in the same position to include the same regions (PET parameters: 3D FOV, 20 cm, ordered subset expectation-maximisation algorithm [OSEM], 3 iterations/12 subsets, full width at half maximum [FWHM], 3 mm). All patients were asked to drink water after the injection and urinate immediately before imaging.

The acquired CT and PET images were transferred to the MedEx system (MedEx Technology Limited Corporation, China) for registration, fusion, and measurement. All [¹⁸F]FAPI-42 and 2-[¹⁸F]FDG PET/CT examinations were independently reviewed by two certified nuclear medicine physicians with over 5 years of experience in nuclear oncology. Any difference in opinion was resolved by consensus. Image interpretation included visual and semiquantitative analyses. For the visualisation of lesions, increasing radioactivity compared with the background uptake (not explained by physiologic uptake) were considered positive lesions. The lesion uptake values were quantified based on the maximum standardised uptake values (SUV_max) for both [¹⁸F]FAPI-42 and 2-[¹⁸F]FDG PET/CT scans, whereas those of normal organs were quantified based on SUV_mean. SUV values were determined by drawing volumes of interest (VOIs) on metastatic lesions observed on [¹⁸F]FAPI-42 and 2-[¹⁸F]FDG PET/CT scans. Circular regions of interest (ROIs) were placed on axial slices around lesions with avid-lesion and were automatically integrated into a 3D VOI (MedEX system). Tumour-to-background ratios (TBRs) were determined to quantify the image contrast and were calculated for local recurrences (relative to the cervical muscle), metastasis in lymph nodes (relative to the cervical muscle), bones (relative to the L5 vertebra spongiosa), and the lungs (relative to the lung parenchyma).

All statistical analyses were performed using the SPSS Statistics (version 25; IBM, Armonk, NY, USA), Excel for Windows (version 15.41; Microsoft, Redmond, Washington, USA), and GraphPad Prism 9.0 (GraphPad Software Corporation) software. The uptake of [¹⁸F]FAPI-42 and 2-[¹⁸F]FDG by normal tissues and positive lesions and TBRs were compared using the Wilcoxon signed-rank test. The uptake values of lesions with different BRAF_V600E mutations were compared using the Mann–Whitney U test. The correlation between the lesion size and SUV_max was analysed via Spearman’s correlation analysis. Two-tailed P-values of < 0.05 were considered statistically significant.

A total of 42 patients (median age, 45 years; range, 12–75 years; 16 [38%] men and 26 [62%] women) underwent [¹⁸F]FAPI-42 PET/CT (Fig. 1); of which, 11 also underwent 2-[¹⁸F]FDG PET/CT within 7 days, and no patient received treatment during this period. All participants underwent neck US and chest CT as per the standard imaging procedure, and their serum Tg and Tg-Ab levels were evaluated during the follow-up period after undergoing [¹⁸F]FAPI-42 PET/CT (range, 2–9 months). Detailed information is presented in Table 1.

All patients tolerated both examinations fairly well, without any drug-related pharmacological effects or physiological responses and related symptoms during injection through the end of the examination.

A total of 161 positive lesions were detected in 27 (64%) patients on [¹⁸F]FAPI-42 PET/CT; of which, 16 (9.9%) were local recurrences at the primary site after thyroidectomy (followed by ¹³¹I ablation), 68 (42.3%) were lymphatic lesions localised in the neck (40/68, 58.8%) and chest (28/68, 41.2%), 52 (32.2%) were pulmonary lesions, and 25 (15.6%) were localised in other sites (bones and pleura). Overall, the mean SUV_max of positive lesions was 3.2, with a TBR of 4.7. The highest uptake intensity was seen in FAPI-positive local recurrences, followed by lymphatic lesions, lesions localised in other sites (bones and pleura), and pulmonary lesions (mean SUV_max: 4.7 versus 3.7 versus 3.0 versus 2.2, respectively; P < 0.0001). Owing to the low background activity, the highest TBR was observed in lesions localised in bones and pleura, followed by local lesions, pulmonary lesions, and lymphatic lesions (mean TBR: 5.7 versus 5.0 versus 4.8 versus 4.18, respectively); however, no significant difference was observed (Fig. 2a). Additionally, the SUV_max of positive lesions showed a moderately positive correlation with lesion diameter (r = 0.327, P < 0.0001) (Fig. 2b).

A total of 38 and 123 positive lesions were detected in patients with elevated TSH levels (eTSH) and those with suppressed TSH levels (sTSH), respectively, on [¹⁸F]FAPI-42 PET/CT. The SUV_max of positive lesions was lower among patients with eTSH than among patients with sTSH (median SUV_max, 2.4 versus 3.2; P = 0.56). However, a significant difference was observed in TBRs between patients with eTSH and sTSH (median TBRs, 3.6 versus 4.2; P = 0.007; Fig. 2e). A total of 79 and 82 lesions were detected in patients with low Tg levels (1–10 ng/mL) and those with high Tg levels (10–500 ng/mL), respectively, on [¹⁸F]FAPI-42 PET/CT. No significant difference was observed in FAPI uptake between patients with low and high Tg levels (median SUV_max, 2.9 versus 2.4; P = 0.0935); however, TBRs were significantly different between the two groups of patients (median TBR, 5.0 versus 3.6; P = 0.002). Moreover, TBRs were higher among patients with positive Tg-Ab than among patients with negative Tg-Ab (median TBR, 5.2 versus 3.6; P = 0.003); however, no significant difference was observed in SUV_max between the two groups (median SUV_max, 2.8 versus 2.6; P = 0.525; Fig. 2e). The activity of background between high and low Tsh level was represented in Supplementary Fig. 1. Furthermore, 16 patients who underwent [¹⁸F]FAPI-42 PET/CT had BRAF_V600E mutation. A total of 65 and 30 positive lesions were detected in patients with BRAF_V600E mutation and wild-type BRAF, respectively. The TBRs of both groups were different (median TBR, 5.0 versus 3.5; P = 0.012); however, SUV_max was not significantly different between the two groups (median SUV_max, 3.0 versus 3.0; P = 0.952) (Fig. 2f).

The maximum intensity projection images of all patients who underwent 2-[¹⁸F]FDG and [¹⁸F]FAPI-42 PET/CT are presented in Fig. 3. The biliary system is an important route for the excretion of [¹⁸F]FAPI-42, unlike 2-[¹⁸F]FDG. The brain, oral muscle, liver and L5 vertebra had lower uptake of [¹⁸F]FAPI-42 than that of 2-[¹⁸F]FDG (mean SUV_mean, 0.1 versus 7.4; 1.4 versus 2.2; 0.9 versus 1.8; 0.8 versus 1.2, respectively; P < 0.05). The detailed parameters of normal tissues are presented in Supplementary Table 1 and Fig. 2a.

Table 2 shows a detailed comparison of SUV_max and TBRs among positive lesions. A total of 90 positive lesions were detected in 7 patients. Moreover, 2 lesions were not detected on 2-[¹⁸F]FDG PET/CT, and 5 were not detected on [¹⁸F]FAPI-42 PET/CT. The SUV_max of 30% (27/90) positive lesions were higher for [¹⁸F]FAPI-42 than for 2-[¹⁸F]FDG, that of 23% (21/90) lesions was equal, and that of 47% (42/90) lesions was lower for [¹⁸F]FAPI-42 than for 2-[¹⁸F]FDG. The SUV_max of positive lesions had a moderately positive correlation between the two tracers (r = 0.651; P < 0.001; Supplementary Fig. 2b). The SUV_max of positive lesions (SUV_max, 2.1 versus 2.6, respectively; P = 0.026; Supplementary Fig. 2c) and pulmonary lesions (SUV_max, 1.3 versus 2.2, respectively; P = 0.002) was significantly different between [¹⁸F]FAPI-42 PET/CT and 2-[¹⁸F]FDG PET/CT. The TBRs of positive lesions (TBR, 3.7 versus 3.8, respectively; P = 0.025) and pulmonary lesions (metastasis to the lung parenchyma) (TBR, 3.3 versus 4.7, respectively; P < 0.001; Supplementary Fig. 2d) were higher on 2-[¹⁸F]FDG PET/CT than on [¹⁸F]FAPI-42 PET/CT. Images representing higher uptake of [¹⁸F]FAPI-42 are shown in Figs. 4 and 5, whereas those representing higher uptake of 2-[¹⁸F]FDG are shown in Fig. 6. On the other hand, patients only underwent [¹⁸F]FAPI-42 PET/CT shown 76 positive lesions, and the distribution of positive lesions had a demonstration at Supplementary Table 2.

In our cohort, thirteen patients are treated by ¹³¹I treatment (dose range: 3.7–7.4 Gbq) after PET/CT. Compared with the post-therapeutic ¹³¹I whole-body scan, [¹⁸F]FAPI-42 PET/CT detected more positive lesions in 4 patients than the ¹³¹I whole-body scan. One patient was shown the same lesions, and five patients were not detected with positive lesions by dual imaging modalities. Unfortunately, three patients showed diffuse pulmonary uptake on post-therapeutic ¹³¹I whole-body scan, but the [¹⁸F]FAPI-42 PET/CT depicted false-negative results in the pulmonary disease of these patients (Supplementary Fig. 3). Among the three patients, one patient manifested positive metastatic lymph nodes by [¹⁸F]FAPI-42 PET/CT, but a post-therapeutic ¹³¹I whole-body scan indicated negative results in metastatic lymph nodes. Another two patients manifested similar results of positive lesions (except pulmonary lesions) by two modalities (Supplementary Table 3).

Fifteen patients in our cohort did not find positive lesions by two imaging modalities. Among these patients, three patients converted to excellent response (ER) according to the 2015 ATA guidelines. Two patients were defined as having an indeterminate response (IR). Ten patients still maintain the situation of biochemical incomplete responses (BIR) until the latest follow-up. All patients have not found structural lesions by CT or US during follow-up (Supplementary Table 4). Representative imaging was presented in Supplementary Fig. 4.