[68Ga] Ga-DOTA-FAPI-04 And [18F] FDG PET / CT For Diagnosis of Metastatic Lesions In Patients With Recurrent Papillary Thyroid Carcinoma

Context: PET CT imaging methods based on broblast activation protein inhibitors (FAPIs) have recently demonstrated promising clinical results. Objective: We aimed to evaluate the use of 68 Ga-FAPI PET / CT and 18 FDG PET / CT imaging techniques to detect the metastatic foci in recurrent papillary thyroid carcinoma. Design and Patients: This is a prospective study. Patients who were diagnosed with papillary thyroid carcinoma, achieved biochemical recovery after the rst operation and having recurrence for papillary thyroid carcinoma on the follow up were included in the study. [ 68 Ga] Ga-DOTA-FAPI-04 and [ 18 F] FDG PET / CT were performed for comparative purpose and detection of recurrence localization. Results: [ 18 F] FDG PET / CT detected the metastatic foci in 21 of 29 patients (72.4%), [ 68 Ga] Ga-DOTA-FAPI-04 was able to detect the metastatic foci in 25 of 29 patients (86.2%). When the two imaging techniques were used together, the metastatic foci in 27 of the 29 patients could be detected (93.1%.). Also between the [ 18 F] FDG PET / CT SUVmax values and [ 68 Ga] Ga-DOTA-FAPI-04 SUVmax values, a statistical signicance was found in favor of 68 Ga-FAPI PET (p = 0,002). Conclusion: In conclusion, 68 Ga-FAPI PET imaging technique can be used as an alternative method to detect the metastatic focus or foci in patients with recurrent papillary thyroid carcinoma. It can also increase the chance of metastatic focus or foci detection when used in conjunction with the 18 FDG PET. serum thyroglobulin levels and negative radioactive iodine (RAI) whole body scans.


Introduction
Conventional treatment of differentiated thyroid carcinomas (DTCs) is total thyroidectomy, radioactive iodine therapy (RAI) and thyroid stimulating hormone (TSH) suppression therapy. With this highly effective treatment approach, 10-year disease-related survival is around 85% in 85% of DTCs. However, in approximately 5% of patients, the tumor may lose its differentiation and iodine uptake ability and may develop metastases. (1,2) There is no point in giving RAI treatment anymore in these patients. In patients who do not respond to tyrosine kinase inhibitors and cytotoxic chemotherapy is currently recommended in poorly differentiated thyroid cancer, but the response rate is quite low.(3) Protease molecules have recently been the subject of studies on many different subjects. There are four enzymatic members of the dipeptidyl peptidase protease molecules (DPP) protein family; broblast activation protein is one of them. (4,5) In studies on FAP, especially in cancer the increase in FAP expression draws attention. Therefore, it has made this protease targetable for therapeutic and diagnostic intervention. FAP expression was found to be signi cantly increased during wound healing, in ammation sites, atherosclerotic plaques, liver brosis and in 90% of epithelial carcinomas. (6) However, there is no clear information about its enzymatic role in different cancer models. (7,8,9) It has been shown that FAP, which is expressed in the cell membranes of active broblasts, plays a role in many enzymatic and / or non-enzymatic pathways in the extracellular matrix and is involved in many protumorogenic pathways that cause tumor progression. ( A patient with malignant or metastatic tissue (one or more lesions) detected in the rst whole body scan after the rst treatment, with no uptake outside the thyroid bed but not retaining I131 in the follow-up, or Tumor tissue that initially has iodine uptake but loses its ability to concentrate radioactive iodine on subsequent scans or treatments, or Patients with metastatic disease and radioactive iodine uptake in some but not all regions, or Patients with metastatic disease and disease progression within 1 year after treatment despite substantial radioiodine therapy.

Radiolabeling Procedure
The radiolabeling process was performed by a fully-automated system without any manual interaction. 68 Ga 3+ was eluted with 0.1 N HCl solution (8.0 ml) followed by passing through the pre-concentrated on a strong cation exchange (SCX) cartridge. The 68 Ga activity was recovered from the SCX cartridge by 0.9 mL eluent (5 M NaCl/HCl(0.1 M)). Reaction vial is lled by 2 ml of H 2 O, 0.4 ml of sodium acetate buffer (pH is around 4.5), 0.2 ml of ethanol and 50 µg of FAPI-04.Than, 68 Ga-activity was transferred to the reaction vial, and it was heated to 95 °C for 10 min. After completion of the reaction, the reaction medium was cooled down and crude product was diluted by adding 5.0 mL of 0.9% NaCl and subsequently puri ed by CM cartridge. Finally, the reaction mixture was passed through a millipore lter (0.22 µm) and was injected intravenously after more than 98% radiochemical purity with 88% radiochemical yield.

FDG
All patients fasted, except for glucose-free oral hydration, for at least 6 h before the IV injection of 370-555 MBq (10-15 mCi) of FDG. At the time of the tracer injection, blood glucose levels were checked and con rmed to be less than 150 mg/dl in all patients.

PET / CT protocol and image evaluation
All patients were examined using a PET/CT system (Discovery™ IQ; GE Healthcare) combining a dedicated, ve-ring PET scanner with LightBurst technology.
PET imaging was performed 60 minutes for 18 FDG and 30 minutes for 68 Ga-FAPI (5-6 mCi) after injection, extending from the vertex to the pelvis, with 5 bed positions of 3 min each. CT images were used for attenuation correction and fusion; no IV contrast medium was used.
The PET/CT images were carefully evaluated by one experienced nuclear medicine physician. PET, CT and fused whole-body images displayed in axial, coronal and sagittal planes were available for review. A semi quantitative analysis of tracer activity was measured as the maximal standardized value uptake (SUVmax) of 18 FDG or 68 Ga-FAPI using the provided software (AW VolumeShare, GE Healthcare).
Images reviewed independently, and two nuclear medicine specialists reviewed the scans independently.

Nature of the metastatic lesion
After detecting uptake with imaging techniques, tyrosine kinase inhibitor was given to some of the patients and the response was followed, and for some patients, the type of tissue involved was determined by biopsy, if possible.

STATISTICAL ANALYSIS
Descriptive statistics of the data obtained from the study are given by mean and standard deviation for numerical variables, and by frequency and percentage analysis for categorical variables. 18 FDG PET SUVmax and 68 Ga FAPI-PET SUVmax variables were evaluated with the normal distribution test Shapiro Wilk test and it was determined that they were not normally distributed (p < 0.05). FDG and FAPI were not normally distributed then median/IQR used. Mann-Whitney U test was used to compare these variables. Analyzes were carried out with the help of SPSS 22.0 program. A signi cance level of p <0.05 was chosen.

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22 of the patients were female, only 7 were male. The disease stages of the patients were calculated according to the TNM staging before the rst operation after diagnosis. The number of sessions of RAI treatment applied in the follow up and the data we consider important for the study are shown in Table 1.   Metastatic focus detection rates of imaging techniques according to disease characteristics are given in Table 3 in detail.

Disscussion
The frequency of thyroid cancer in the world is gradually increasing with the development of imaging techniques. Thyroid cancer is now detected much earlier and in much smaller sizes. (12) It is expected to be the 2nd or 3rd most common cancer in the 2020s. Preoperative use of 18 FDG PET / CT is still controversial. (14,15) But the patient group in our study is a high-risk patient group with recurrence despite post-operative radioactive iodine treatments. Therefore, it seems reasonable to use 18 FDG PET / CT in such patients where conventional techniques are insu cient. However, despite the use of 18 FDG PET / CT, there are still patients whose metastatic focus or foci cannot be detected hence who cannot be cured. In such cases, a new imaging technique that could help the clinicians will come to the fore to guide the management. Therefore, in our study, we focused on the use of 68 Ga-FAPI in patients with recurrent thyroid cancer.
In a multi-center study conducted by Kratochwil, the use of 68 Ga-FAPI-PET / CT in 28 different cancer types was examined. Six of the 80 patients in this study were patients with differentiated thyroid cancer, and the histological analysis or subgroups in the study were unknown. Although low SUVmax values were detected, imaging with 68 Ga-FAPI PET / CT was detected in differentiated thyroid cancer. The fact that our patient group is advanced recurrent patients may explain our higher SUVmax values in 68 Ga-FAPI imaging. Also, cancer-associated broblasts and extracellular brosis may increase in desmoplastic tumors, leaving the original tumor cells in the minority. (4) According to the pathological classi cation of papillary thyroid carcinoma; The classic variant and follicular variant that exist in our patient groups are well differentiated, while the tall cell variant is moderately differentiated.
(16) With all these recurrence and tumor differentiation features, it was shown in our study that 68 Ga-FAPI was not inferior to 18  We observed that in both imaging techniques, as the TG value increases, the rate of metastasis detection has increased also. 68 Ga-FAPI PET imaging was able to detect all metastasis foci when TG> 300 (in all 11 patients). It has been suggested that low TG in patients with recurrent thyroid carcinoma may be associated with tumor dedifferentiation. (19) Regarding this, at lower TG levels (Thyroglobulin antibody negative), 18 FDG PET detected the foci in 3 of 4 patients and 68 Ga-FAPI PET was able to detect the foci in 2 of 4 patients.
There are studies showing that 68 Ga-FAPI PET imaging is more speci c in tumoral conditions because of the low levels of expression of broblast activated protein in the body, but high expression of cancer-associated broblasts in the presence of tumor cells. (20,21,22) According to the results of our study, using both imaging techniques together seems more useful in detecting the metastatic focus or fuci in di cult cases such as recurrent papillary thyroid cancer.
The limitations of our study are as follows; the low number of patients in the study because the study was conducted with a speci c disease group and histopathological con rmation of metastases could not be made consequent to their anatomical localization.
In conclusion, 68 Ga-FAPI PET imaging technique can be used as an alternative method to detect the metastatic focus or foci in patients with recurrent papillary thyroid carcinoma. It can also increase the chance of metastatic focus or foci detection when used in conjunction with the 18 FDG PET.