Comparison of [68Ga]Ga-DOTA-FAPI-04 PET/CT and [18F]FDG PET/CT for diagnosis of metastasis in differentiated thyroid cancer with negative iodine scintigraphy


 Purpose Evaluation of the potential value of [68Ga]Ga-DOTA-FAPI-04 positron emission tomography/computed tomography (PET/CT) and [18F]FDG PET/CT in detection of differentiated thyroid cancer (DTC) metastases. Methods [68Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT scans were performed on suspected metastatic DTC patients. Maximum standard uptake value (SUVmax) was used to quantify the uptake of positive lesions. Histopathology or follow-up images were used as the standard for final diagnosis.Results A total of 35 DTC patients underwent [68Ga]Ga-DOTA-FAPI-04 and [18F]FDG PET/CT scans in the same time period. Average SUVmax of [68Ga]Ga-DOTA-FAPI-04 uptake by DTC lesions was low (average SUVmax<7). The difference in detection rate of SUVmax of[68Ga]Ga-DOTA-FAPI-04compared with that of[18F]FDG in bone metastases was statistically significant (P=0.000, 0.049). There were differences in the detection rate of other distant and lymph node metastases, and SUVmax, with no statistical significance (P>0.05). The specificity, accuracy, and positive predictive value of [68Ga]Ga-DOTA-FAPI-04 in the diagnosis of lymph node lesions were higher than those of [18F]FDG PET/CT, and the difference was statistically significant (c2=16.583，c2=9.910，c2=7.548，c2=2.781, P＜0.05). There were no statistically significant differences in sensitivity, specificity, accuracy, and positive and negative predictive value of the two groups in the diagnosis of distant metastases (c2=0.440，c2=4.956，c2=0.013，c2=1.194，c2=2.618，P＞0.05). Conclusion [68Ga]Ga-DOTA-FAPI-04 has certain advantages over [18F]FDG PET/CT in detecting lymph node and bone metastases. It has the same ability to detect other distant metastases and can be used as a supplementary imaging method for DTC.


Introduction
Differentiated thyroid cancer (DTC) is one of the tumours with the best prognosis in malignant tumours.
However, the overall risk of recurrence is about 20% [1]. In case of suspicion of DTC persistence/recurrence, the lack of consistency between negative routine imaging tests and high or gradually increasing serum thyroglobulin (Tg) levels prompted attempts to detect DTC recurrence and metastasis.
Fibroblast activation protein (FAP) is highly expressed in cancer associated broblasts (CAFs) in more than 90% of malignant epithelial tumours [2].It promotes the disease progression in different types of cancers, thereby, worsening the prognosis [3]. Therefore, FAP is considered to be a promising target that can be used for radionuclide-based tumour diagnosis and treatment [4]. FAP-inhibitors (FAPI) are caused in tumours with characteristics of high uptake and very low accumulation in normal tissues and rapid circulatory clearance [5]. Studies have shown that [ 68 Ga]Ga-DOTA-FAPI-04 positron emission tomography/computed tomography (PET/CT) has advantages in the detection of initial staging, recurrence, and metastasis of some tumours [6][7][8].Some studies reported that [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT has limited research value in the recurrence and metastasis of DTC [7,9], however,the sample size of the above-mentioned literature studies is small. Therefore, the purpose of this study is to compare

Selection of patients
This prospective study was approved by our hospital's Institutional Review Board (AHSWMU-2020-035).
After approval and determination of the eligibility of the patient by an oncologist and was conducted in accordance with the 1964 Helsinki Declaration and its subsequent revisions or similar ethical standards.
between March 2020 to July 2020, patients were enrolled in our institute for participation in this study.
The inclusion criteria were as follows: (i)All patients who underwent total thyroidectomy, with or without neck lymph node dissection, followed by oral iodine-131 ( 131 I) for ablative treatment; (ii)Elevated Tg and/or anti-thyroglobulin autoantibody (TgAb) level at clinical follow-up, and indication of lymph node metastasis on neck colour Doppler ultrasound or clinically suspected metastasis; (iii)postoperative time>6 months; (iv)age>18 years old; (v)Patients who received [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT additionally; (vi)Patients who provided informed consent (signed by the participant, parent, or legal representative) and consent in accordance with the guidelines of the clinical research ethics committee.
The exclusion criteria were: (i)Patients during pregnancy and lactation; (ii)Patients with liver and kidney dysfunction; (iii)Study participants, parents, or legal representatives who were unable or unwilling to provide written informed consent. Criteria for diagnosis: Histopathological results were considered as the gold standard for diagnosis. In case of no pathological diagnosis, follow-up results, including imaging, laboratory examination results, and clinical comprehensive judgment (follow-up time ≥ 6 months) were considered. Recurrence or metastasis was diagnosed during follow-up based on the following criteria: (i)Typical imaging features of malignancy on CT, MRI, and neck ultrasound; (ii)Malignant progression of the lesion occurring in the later follow-ups, and gradual increase in serum Tg or TgAb levels; iii After treatment with 131 I, radiotherapy, or targeted therapy, the size of the lesion reduced signi cantly or the disease stabilized without further progression.
All patients underwent both PET/CT scans within 3 days. patient was required to fast for more than six hours, maintain blood glucose less than 11.1moml/L, and take rest for one hour. The PET/CT scanner of China United Imaging Corporation was then used for imaging. The above preparations were not required before a[ 68 Ga]Ga-DOTA-FAPI-04 PET/CT scan. First CT scan (scanning range from the top of the skull to the middle and lower 1/3rd of the femur) was done at a tube voltage of120kv, tube current of 100mA, and layer thickness of5.0mm; followed by PET scan in differingopinions. The lesions with activity exceeding that of the adjacent background tissue were marked as positive and the cross-section with the highest metabolism was selected to record the maximum standardized uptake value (SUVmax) of the lesion. If the number of lesions in a certain organ was not clear on recording the SUVmax, mean, and sum of the positive lesions at each site scope, the organ was excluded. According to the stage and anatomical structure of differentiated thyroid cancer, the lymph nodes weredivided into cervical area VI (pretracheal, paratracheal, and prelaryngeal lymph nodes), and other areas of the neck (I-V Regional lymph nodes or supraclavicular fossa), mediastinum (including epicardium, bilateral internal mammary area, etc.), axilla, abdomen (including para-aortic, hilar, retroperitoneum, abdominal cavity), and inguinal lymph nodes. Brain, lung, liver, bone, and other involvements were classi ed as separate parts.

Statistical analyses
All statistical analysis was done using SPSS 25.0 software. Continuous data were expressed as mean ± standard deviation and categorical data were expressed as percentages. The Wilcoxon paired signed-rank test was used to compare the SUVmax values of [ 18 F]FDG and [ 68 Ga]Ga-DOTA-FAPI-04, and to assess if their difference was statistically signi cant. A comparison of the results of PET/CT visual interpretation with the results of histopathology (biopsy or surgery) was used as the gold standard for diagnosis. The detection rate was compared using Fisher's exact probability method. Comparisons of the sensitivity, speci city, accuracy, and positive-(PPV) and negative predictive value (NPV) of [ 18 F]FDG and [ 68 Ga]Ga-DOTA-FAPI-04 were done for evaluation of the diagnostic effect. Two-tailed p values of less than 0.05 were considered statistically signi cant.

Results
Characteristics of the patients Between March 2020 to July 2020, a total of 46 DTC patients were enrolled in the initial stage of the study; of these 11 patients were excluded from analysis due to the following reasons:7 patients were In this study, very few patients showed positive lymph nodes in the inguinal and axillary areas, hence, PET-positive lymph nodes were divided depending on 4 locations: the central area of the neck (pretracheal, paratracheal, and prelaryngeal lymph nodes), other areas of the neck (the I-V area lymph nodes or supraclavicular fossa), mediastinum (including epicardium, internal mammary area, etc.), and abdomen (including abdominal aorta, hilar, retroperitoneum, abdominal cavity). As shown in Table 1, there was no statistically signi cant difference in SUVmax uptake in positive lymph nodes between the two groups (P=0.054). Speci c analysis of SUVmax uptake of each regional lymph node found that the  Table 2). However, the detection rate difference between the two groups was not statistically signi cant (P=0.774) ( Table 3). A representative case showing the different SUVs of lymph nodes metastasis in the two types of scans is presented in    Fig.3. Fig.4.

Discussion
The purpose of this prospective study was to compare the potential value of [ 68 Ga]Ga-DOTA-FAPI-04 PET/CT and [ 18 F]FDG PET/CT in the detection of DTC disease metastases.
Differentiated thyroid cancer (DTC) is the commonest thyroid cancer (>90% of all thyroid cancers). In the majority of cases, the prognosis is excellent with 97% cause-speci c 10-year survival. Because DTC is not sensitive to radiotherapy and chemotherapy, 131 I is the rst choice for adjuvant therapy after surgery, especially for patients with metastases [10]. However, about 33%-50% of patients with DTC metastasis eventually progress to radioiodine refractory during the course of the disease. Such patients generally have a poor prognosis, with a survival period of only 3-6 years [11]. In order to optimize the bene t of each patient's treatment, while minimizing the treatment risks associated with radiation exposure, early identi cation of suitable treatment options for DTC patients and formulation of appropriate treatment strategies from a long-term perspective should be done. It will help improve long-term expectations of DTC patients.
[ 18 F]FDG PET imaging plays an essential role in the diagnosis and management of varieties of malignancy; [ 18 F]FDG PET/CT is suitable for DTC patients with 131 I-WBS negative and elevated Tg [12]. In contrast, patients with well-differentiated DTC subtypes have lower sensitivity [13]. [ 68 Ga]Ga-DOTA-FAPI-04 has the advantages of shorter half-life and less injection dose, which can shorten the waiting and scanning time of patients after intravenous injection, and reduce the burden of inspection radiation [5]. According to the results of later pathological tissue biopsy and follow-up, it is considered that more in ammatory lymph nodes take up FDG, resulting in lower speci city. This may be because DTC lymph node metastasis is usually located in the neck, which is also a popular area for in ammatory lymph nodes. Therefore, the false positive rate of [ 18 F]FDG increases, which is consistent with previous literature reports [15].  antibody against FAP has been tested in preclinical experiments and proved to be effective [21]. Targeting     The aky symmetrical [68Ga]Ga-DOTA-FAPI-04 uptake of the bilateral breast is increased (a, black arrow), and benign breast disease (mammary hyperplasia) is considered after Ultrasonography.