Head-to-Head Comparison of [ 68 Ga]Ga-DOTA.SA.FAPi and [ 68 Ga]Ga-DOTANOC Positron Emission Tomography/Computed Tomography Imaging for the Follow-Up Surveillance of Patients with Medullary Thyroid Cancer

Abstract

Background:

A theranostic probe for accurate staging and treatment is crucial for the management of medullary thyroid cancers (MTCs). The abundance of stroma in most of thyroid cancers, including MTC, opens new avenues for selecting cancer-associated fibroblasts (CAFs) as new molecular imaging and therapeutic targets. [⁶⁸Ga]Ga-labeled fibroblast activation protein inhibitor (FAPi) molecules have gained importance as alternative molecular imaging agents in the imaging of thyroid cancers. The purpose of this study was to compare the detection efficiency of primary and metastatic lesions of MTCs between [⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DOTANOC positron emission tomography (PET) radiotracers.

Materials and Methods:

In this retrospective study, [⁶⁸Ga]Ga-DOTANOC and [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT (computed tomography) images were compared using patient-based and lesion-based analysis in patients with MTC for follow-up assessment. The quantitative assessment included comparing standardized uptake values corrected for lean body mass (SULpeak) and tumor-to-background ratios (TBR). The findings on both scans were validated with the morphological findings of the diagnostic CT.

Results:

Twenty-seven patients (21 males and 6 females) with a mean age of 42.4 ± 13.2 years (range 14–66 years) were included in the study. [⁶⁸Ga]Ga-DOTA.SA.FAPi had similar sensitivities as that of [⁶⁸Ga]Ga-DOTANOC PET/CT for detecting primary tumors (100% [18 of 18] vs. 94.4% [17 of 18], p = 0.979) involved lymph nodes (98.3% [118 of 120] vs. 95% [114 of 120], p = 0.288), and brain metastases (100%). [⁶⁸Ga]Ga-DOTA.SA.FAPi demonstrated significantly higher sensitivities than [⁶⁸Ga]Ga-DOTANOC PET/CT for detecting lung nodules (93.5% [87 of 93] vs. 68.9% [64 of 93], p < 0.0001), liver (100% [105 of 105] vs. 46.4% [49 of 105], p < 0.0001), bone (92.4% [110 of 119] vs. 76.5% [91 of 119], p = 0.001), and pleural metastases 98.2% versus 0%. Higher uptake values and TBR values were reported with [⁶⁸Ga]Ga-DOTA.SA.FAPi compared with that of [⁶⁸Ga]Ga-DOTANOC.

Conclusion:

[⁶⁸Ga]Ga-DOTA.SA.FAPi outperformed [⁶⁸Ga]Ga-DOTANOC PET/CT in the detection of distant metastases with both patient-based and lesion-based analysis in MTCs.

Introduction

Medullary thyroid cancer (MTC) is a type of neuroendocrine tumor (NET) derived from the thyroid C cells and accounts for <5% of thyroid cancers.^1,2 MTCs do not often accumulate radioiodine and hence not responsive to radioiodine treatment. In this subtype of thyroid cancer, positron emission tomography (PET) imaging modalities, including [¹⁸F]F-FDG, [¹⁸F]F-Fluorodopa (FDOPA), and [⁶⁸Ga]Ga-DOTA-labeled somatostatin analogs (SSAs), are used in the initial diagnosis, restaging, treatment assessment, and follow-up surveillance.^3,4

As compared with the other NETs, the tumor detection rate on somatostatin receptor (SSTR)-based PET or PET/CT (computed tomography) in recurrent MTC is only 63.5%, as depicted in a meta-analysis by Treglia et al. And as per the European Association of Nuclear Medicine (EANM) practice guideline for PET/CT imaging in medullary carcinoma thyroid, SSTR PET/CT needs to be considered only in cases with inconclusive imaging results with [¹⁸F]F-FDOPA, and [¹⁸F]F-FDG PET/CT results and to assess the feasibility of peptide receptor radionuclide therapy (PRRT).³

Currently, there is no ideal PET tracer with an optimal detection rate for MTC. Hence, developing theranostic probes with promising detection rates and with a possible therapeutic dimension is crucial and could be a potential added value for the substantial improvement of the management of MTCs.

Activated fibroblasts, that is, cancer-associated fibroblasts (CAFs) comprise the crucial components of the tumor microenvironment (TME) and are abundantly expressed in various subtypes of thyroid cancers, including MTC.^5
–7 The bidirectional interaction of the CAFs with the tumor cells facilitates new imaging and therapeutic targets.^5,7 Recent investigations with gallium-68 labeled fibroblast activation protein (FAP) inhibitors have demonstrated a significant theranostic role of fibroblast activation protein inhibitor (FAPi) in various cancers, including thyroid cancer,⁸ but not much has been explored on its role in MTC. In this retrospective study, we aimed to conduct a head-to-head comparison of [⁶⁸Ga]Ga-DOTA.SA.FAPi with [⁶⁸Ga]Ga-DOTANOC PET/CT imaging for the follow-up surveillance of patients with MTC.

Materials and Methods

This retrospective study was approved by the “All India Institute of Medical Sciences institute ethics committee,” New Delhi, India, and was completed in accordance with the Helsinki Declaration (Ref. No. IECPG: 22/27). Written informed consent was obtained from all patients who participated in the study, for the imaging, use of clinical information to analyze data, and use of images for publication purposes. This study is a collaboration between the Department of Nuclear Medicine, Medical Oncology at All India Institute of Medical Sciences, and the Department of Chemistry, Johannes Gutenberg University, Mainz, Germany, which provided the labeling precursor as depicted in Figure 1.

FIG. 1.

Chemical structure of the labeling precursor DOTA.SA.FAPi. FAPi, fibroblast activation protein inhibitor.

Patients fulfilling the following eligibility criteria were enrolled in the study: patients with histologically proven MTC, and patients who underwent both [⁶⁸Ga]Ga-DOTANOC and [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT within a time interval of 1 month, and patients who did not receive any anticancer treatment within 4 weeks. Patients with a known inflammatory condition, dual malignancies, pregnant patients, and those unwilling to undergo two PET/CT scans were excluded from the study. According to the eligibility criteria, 27 patients were included in the study (Fig. 2).

FIG. 2.

Patient flow diagram.

Synthesis of [⁶⁸Ga]Ga-DOTA.SA.FAPi and quality control

Radiolabeling of [⁶⁸Ga]Ga-DOTA.SA.FAPi was conducted as detailed in our previous publications.⁸

PET/CT acquisition

The mean injection doses of [⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DOTANOC were 185 MBq. After 60 minutes of intravenous administration of both radiotracers, scans were obtained on a 128-slice GE Discovery 710* 128 Slice PET/CT Scanner with a 40 mm detector at a 0.35-second rotation speed. The patient was in the supine position for all acquisitions. The scans included a CT and a PET scan after a preliminary scout image, to establish the field of view. A diagnostic dosage CT with parameters of 300–380 mAs, 120 kVp, slice thickness 3.75 mm, and pitch 0.6 was used for the CT scan. In patients with brain metastases, spot images were obtained with a slice thickness of 1.25 mm on CT at 120 kVp, 300–380 mAs, and a pitch of 0.6, as needed. A GE Xeleris workstation was used for image processing and analysis. For [⁶⁸Ga]Ga-DOTANOC and [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT, image capture, and analysis entailed a qualitative and quantitative comparison of the tracers.

Data interpretation

To assess the diagnostic ability of both the radiotracers, patient-based and lesion-based analyses were conducted in both primary and metastatic lesions. Two nuclear medicine physicians with more than 15 years of expertise in interpreting PET/CT scan results independently evaluated and processed both [⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DOTANOC PET/CT scans. They were unaware of the patients' clinical history and histopathological examinations (HPE) status. Any discrepancies in the interpretation were discussed and resolved by consensus.

Data analysis and processing

The qualitative analysis included a visual judgment of radiotracer uptake, which was validated by morphological findings on CT, the reference standard. When radiopharmaceutical uptake was greater than the background, a lesion was considered positive on PET. The uptake in the lesions on both scans were compared with the CT counterpart's morphological features/characteristics. A three-dimensional auto-contour region of interest (ROI) at a 40% SULpeak threshold was carefully drawn around the site of [⁶⁸Ga]Ga-DOTANOC and [⁶⁸Ga]Ga-DOTA.SA.FAPi expressing lesions on transaxial images for quantitative comparisons.

To quantitatively compare the uptake in the lesions between the radiotracers, the ROIs were presented as standardized uptake value (SUV) corrected for lean body mass: SULpeak. The SUV values (peak, average, median, and range) for both [⁶⁸Ga]Ga-DOTANOC and [⁶⁸Ga]Ga-DOTA.SA.FAPi were recorded for each site. The tumor-to-background ratio (TBR) was calculated by dividing the SULpeak of the primary tumor/metastases by the corresponding background SULpeak values.

Definitions

True-positive lesion

Uptake in the lesion seen on [⁶⁸Ga]Ga-DOTANOC/[⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT images were higher than the background and were found to be positive on diagnostic CT/histological examination.

False-positive lesion

Uptake in the lesion seen on [⁶⁸Ga]Ga-DOTANOC/[⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT images were higher than the background and were found to be negative on diagnostic CT/histological examination.

True-negative lesion

No uptake seen on [⁶⁸Ga]Ga-DOTANOC/[⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT images were higher than the background and the results on diagnostic CT/histological examination.

False-negative lesion

Lesions that were missed in [⁶⁸Ga]Ga-DOTANOC/[⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT images higher than the background were found to be positive for malignancy at diagnostic CT/histological examination.

Statistical analysis

Statistical analysis was performed using MedCalc statistical software (v15.0). Continuous variables were presented in terms of mean, median, standard deviation, range, and interquartile range (IQR). [⁶⁸Ga]Ga-DOTANOC and [⁶⁸Ga]Ga-DOTA.SA.FAPi uptakes were compared using Wilcoxon signed-rank test. The sensitivities of [⁶⁸Ga]Ga-DOTANOC and [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT examinations were calculated and compared. p-Values ≤0.05 were considered significant. Indeterminate results from the [⁶⁸Ga]Ga-DOTA.SA.FAPi and [¹⁸F]F-FDG PET/CT tests were approached as false-negative (FN) or false-positive using a diagnostic quality CT scan as the reference standard. Participants with missing data, including histopathology, tests, or reference standard imaging, were excluded from the analysis (Fig. 2).

Results

Twenty-seven patients with MCT (21 males and 6 females) with a mean age of 42.4 ± 13.2 years (range 14–66 years) were found eligible and were included in the study analysis. Post-intravenous injection of [⁶⁸Ga]Ga-DOTA.SA.FAPi, all vital parameters remained normal and no adverse events were noted. All the patients underwent a minimum of two lines of prior treatments. The median serum calcitonin levels at the time of imaging were 666.5 pg/mL (25–75% IQR: 389–1145 pg/mL). Twenty-four patients underwent total thyroidectomy and nodal dissection. Fourteen patients (52%) previously received kinase inhibitors, four took mammalian target of rapamycin (mTOR) inhibitors, six patients received local radiotherapy to the neck, seven received palliative radiotherapy to the distant metastatic sites, and four patients received [¹⁷⁷Lu]Lu-DOTATATE therapy (Table 1).

Table 1.

Demographics and Clinical Characteristics of Patients

Parameters	Values (N = 27)
Age in years (mean ± SD; range)	42.4 ± 13.2 years (14–66)
Sex
Male	21 (77.7%)
Female	6 (22.3%)
MTC subtype
Sporadic	25 (92%)
MEN 2A	1 (4%)
MEN2B	1 (4%)
Primary treatment
Total thyroidectomy+nodal dissection	24 (88.8%)
Nodal dissection	3 (6%)
TKIs	14 (52%)
One line	8 (57%)
Two lines	6 (43%)
mTOR inhibitors	4 (15%)
¹⁷⁷Lu-DOTATATE therapy	4 (15%)
External beam radiation therapy	13 (%)
Primary site (neck)	6 (46%)
Metastatic site	7 (54%)
Baseline median serum calcitonin values (pg/mL), median, 25–75% IQR	666.5 (389–1145)

IQR, interquartile range; MTC, medullary thyroid cancer; mTOR, mammalian target of rapamycin; SD, standard deviation; TKI, tyrosine kinase inhibitor.

Comparison of lesion detection and uptake between [⁶⁸Ga]Ga-DOTANOC and [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT scans

Primary tumor

Among the 27 patients, 18 remnants were identified on diagnostic CT. Intense uptake of [⁶⁸Ga]Ga-DOTA.SA.FAPi was noted in all the 18 lesions (100%). However, [⁶⁸Ga]Ga-DOTANOC identified/showed expression in 17 lesions (94.4%, confidence interval [CI]: 78.2–99.5%; p = 0.979). The median SULpeak values were comparable between the tracers (SULpeak [⁶⁸Ga]Ga-DOTA.SA.FAPi: 6.5; [IQR: 5–8.5], vs. [⁶⁸Ga]Ga-DOTANOC: 5.3; [IQR: 5.1–7.5, p = 0.206]) (Table 2). The TBR was significantly higher in [⁶⁸Ga]Ga-DOTA.SA.FAPi than in [⁶⁸Ga]Ga-DOTANOC (SULpeak; [⁶⁸Ga]Ga-DOTA.SA.FAPi: 5.5 [IQR: 4.7–7.7] vs. [⁶⁸Ga]Ga-DOTANOC: 1.3 [IQR: 0.1–1.5]; p < 0.0001).

Table 2.

Comparison of Various Parameters Between [⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DOTANOC Positron Emission Tomography/Computed Tomography Imaging

Parameters	Imaging method	Primary	Lymph node metastasis	Lung metastasis	Pleural thickening	Liver metastases	Bone metastases	Brain metastases
Patient-based analysis
	CT	16	22	16	2	13	12	3
	[⁶⁸Ga]Ga-DOTA.SA.FAPi	16 (100%)	21 (95.4%)	13 (81.3%)	2 (100%)	13 (100%)	11 (91.6%)	3 (100%)
	[⁶⁸Ga]Ga-DOTANOC	15 (93.7%)	20 (91%)	13 (81.3%)	1 (50%)	10 (77%)	10 (83.3%)	3 (100%)
	p	0.968	0.835	0.650	0.673	0.221	0.998	1.000
Lesion-based analysis
	CT	18	120	93	3	105	119	11
	[⁶⁸Ga]Ga-DOTA.SA.FAPi	18 (100%)	118 (98.3%)	87 (93.5%)	2 (98.2)	105 (100%)	110 (92.4%)	11 (100%)
	[⁶⁸Ga]Ga-DOTANOC	17 (94.4%)	114 (95%)	64 (68.9%)	0	49 (46.6%)	91 (76.5%)	11 (100%)
	p	0.979	0.288	<0.0001	^*	<0.0001	0.001	1.000
SULpeak
	[⁶⁸Ga]Ga-DOTA.SA.FAPi	6.5 (5–8.5)	6.9 (5.9–9)	4.6 (2.3–9)	4.3 (2–6.6)	6 (5.4–11.2)	5.8 (4.5–11.6)	6 (3.4–6.9)
	[⁶⁸Ga]Ga-DOTANOC	5.3 (5.1–7.5)	6 (5.4–8.1)	2.1 (1.9– 7.2)	2.1 (1.9–5.1)	2.2 (0.4–5.7)	1.95 (1.6–3.9)	4.2 (2.1–6.2)
	p	0.206	0.031	0.012	0.0001	0.041	0.114	0.130
TBR (SUVpeak)
	[⁶⁸Ga]Ga-DOTA.SA.FAPi	5.5 (4.7–7.7)	5.8 (0–8.1)	1.6 (1.1–4.08)	3.2 (1.1–4.2)	4.8 (3.1–5.5)	5.8 (4.6–9.7)	12 (1.9–14.2)
	[⁶⁸Ga]Ga-DOTANOC	1.3 (0.1–1.5)	4.1 (0–5.2)	1.8 (1.1–2.9)	1.6 (0.6–2.3)	0.7 (0.3–1.4)	1.5 (0.7–2.1)	8 (5.2–10.9)
	p	<0.0001	0.003	0.357	0.002	0.031	0.0002	0.145

Not done.

CT, computed tomography; FAPi, fibroblast activation protein inhibitor; SULpeak, peak SUL (standardized uptake value corrected for lean body mass) in a spherical 1-cc Volume of Interest; SUV, standardized uptake value; TBR, tumor-to-background ratio.

Lymph node metastases

Twenty-two (81.5%) out of 27 patients had a total of 120 lymph node (LN) metastasis on CT. While [⁶⁸Ga]Ga-DOTA.SA.FAPi correctly diagnosed 98.3% (118/120; CI: 94–99.7%) of LN metastases (true-positive [TP]: 118, FN: 2), [⁶⁸Ga]Ga-DOTANOC diagnosed 95% (114/120; CI: 89.4–98%; TP: 114, FN: 6); p = 0.288. However, The SULpeak and TBR values of the LNs were significantly higher for [⁶⁸Ga]Ga-DOTA.SA.FAPi (SULpeak; [⁶⁸Ga]Ga-DOTA.SA.FAPi: 6.9 [IQR: 5.9–9] vs. [⁶⁸Ga]Ga-DOTANOC: 6 [IQR: 5.4–8.1]; p = 0.031; TBR SULpeak; 5.8 [IQR: 0–8.1] vs. 4.1 [IQR: 0–5.2]; p = 0.003) (Table 2).

Lung metastases

Lung metastases were detected on CT in 16 patients (59.3%). Both [⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DOTANOC showed concordant findings in 13 (81.3%) patients. On detailed lesion-based analysis, out of 93 lung nodules identified on CT, [⁶⁸Ga]Ga-DOTA.SA.FAPi diagnosed 87 (93.5%, CI: 86.4–97.5%) while [⁶⁸Ga]Ga-DOTANOC diagnosed 64 (68.9%; CI: 58.4–78%) lesions (p < 0.0001). Similarly, SULpeak and TBR uptake parameters were remarkably higher on [⁶⁸Ga]Ga-DOTA.SA.FAPi PET compared with that of [⁶⁸Ga]Ga-DOTANOC (Table 2).

Liver metastases

Thirteen (48.2%) patients out of 27 patients had liver metastasis on CT and [⁶⁸Ga]Ga-DOTA.SA.FAPi has shown completely concordant findings in all 13 (100%) patients. In contrast, [⁶⁸Ga]Ga-DOTANOC detected liver metastases in 77% (10/13; CI: 67.7–84.6%) patients. [⁶⁸Ga]Ga-DOTA.SA.FAPi correctly diagnosed all the liver lesions (105/105), but [⁶⁸Ga]Ga-DOTANOC could detect only 46.6% (49/105; CI: 36.8–56.5%) liver lesions (p < 0.0001) (Figs. 3 and 4), among whom [⁶⁸Ga]Ga-DOTANOC completely failed to detect any liver metastases in two patients (Fig. 3). The SULpeak of the liver lesions was higher in [⁶⁸Ga]Ga-DOTA.SA.FAPi than that in [⁶⁸Ga]Ga-DOTANOC (SULpeak; [⁶⁸Ga]Ga-DOTA.SA.FAPi: 6 [IQR: 5.4–11.2] vs. [⁶⁸Ga]Ga-DOTANOC: 2.2 [IQR: 0.4–5.7]; p = 0.041)(Table 2). Similarly, the TBR values were significantly higher (p = 0.031) with [⁶⁸Ga]Ga-DOTA.SA.FAPi: 4.8 (IQR: 3.1–5.5) compared with that of [⁶⁸Ga]Ga-DOTANOC: 0.7 (IQR: 0.3–1.4).

FIG. 3.

An 18-year-old female patient presented with a painless neck swelling, which on evaluation by fine needle aspiration cytology was suspicious of medullary carcinoma thyroid. Biochemically baseline serum calcitonin level and CEA levels were 54,800 pg/mL and 3909 ng/mL, respectively. The baseline [⁶⁸Ga]Ga-DOTANOC PET/CT was performed for metastatic workup, which revealed the neck primary with bilateral neck and mediastinal lymph nodes with a few sclerotic skeletal lesions with no SSTR expression (A). As the tumor markers were raised and few lesions were nonavid on DOTANOC PET/CT, [¹⁸F]-FDG PET/CT (B) was performed, which revealed no additional metabolically active lesions. The patient underwent total thyroidectomy with bilateral radical neck dissection. As the postoperative calcitonin (24,080 pg/mL) was elevated, [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT was performed. [⁶⁸Ga]Ga-DOTA.SA.FAPi (C) maximum intensity images clearly demonstrated multiple liver metastases (blue arrow) and skeletal metastasis (yellow arrow) with FAP expression. The high hepatic background in both DOTANOC and FDG PET/CT might be a possible explanation for the nonvisualization of the lesions, which was not the case with FAPi biodistribution. An additional finding in the uterus of the patient who was menstruating at the time of the [⁶⁸Ga]Ga-DOTA.SA.FAPi PET was the intense uptake in the uterine cavity (white arrow). CEA, carcinoembryonic antigen; CT, computed tomography; FAP, fibroblast activation protein; PET, positron emission tomography; SSTR, somatostatin receptor.

FIG. 4.

A 25-year-old male with biopsy-proven medullary thyroid carcinoma was referred for baseline staging. [⁶⁸Ga]Ga-DOTANOC PET/CT (A) performed postoperatively revealed primary neck lesion, bilateral cervical, mediastinal, and abdominal lymph nodes, and liver lesions. As compared with [⁶⁸Ga]Ga-DOTANOC PET, diagnostic CT (B), [⁶⁸Ga]Ga-DOTA.SA.FAPi (C) revealed a greater number of liver lesions with good tumor-to-background contrast and also a few skeletal lesions with FAPi uptake (non-DOTANOC avid; blue arrow).

Bone metastases

Bone lesions were classified according to the number of bone metastasis (Table 2). Among the 12 (44.4%) patients with radiological bone metastases, these were diagnosed in 91.6% (11/12; CI: 61.4–99.7%) of patients with [⁶⁸Ga]Ga-DOTA.SA.FAPi and 83.3% (10/12; CI: 51.5–97.9%) patients (p = 0.998) with [⁶⁸Ga]Ga-DOTANOC PET/CT. Out of 119 bone lesions seen in CT, [⁶⁸Ga]Ga-DOTA.SA.FAPi diagnosed more lesions than [⁶⁸Ga]Ga-DOTANOC {Number of bone lesions; [⁶⁸Ga]Ga-DOTA.SA.FAPi: 110 (92.4%, CI: 86.6–96%) vs. [⁶⁸Ga]Ga-DOTANOC: 91 (76.5%, CI: 67.8–83.7%), p = 0.001} (Figs. 3 and 4). The SULpeak values for bone lesions were 5.8 (IQR: 4.5–11.6) with [⁶⁸Ga]Ga-DOTA.SA.FAPi and 1.9 (IQR: 1.6–3.9) with [⁶⁸Ga]Ga-DOTANOC (p = 0.114). However, TBR values were significantly higher in ⁶⁸Ga]Ga-DOTA.SA.FAPi compared with [⁶⁸Ga]Ga-DOTANOC {SULpeak; [⁶⁸Ga]Ga-DOTA.SA.FAPi: 5.8 (IQR: 4.6–9.7) vs. [⁶⁸Ga]Ga-DOTANOC: 1.5 (IQR: 0.7–2.1); p = 0.0002} (Table 2).

Brain metastases

Both radiotracers had completely concordant findings with the radiological findings in all three patients with brain metastasis (3/3) in all 11 brain lesions. The SULpeak values were not different between the radiotracers (SULpeak; [⁶⁸Ga]Ga-DOTA.SA.FAPi: 6 [IQR: 3.4–6.9] vs. [⁶⁸Ga]Ga-DOTANOC: 4.2 [IQR: 2.1–6.2]; p = 0.130). However, The TBR values are 12 (IQR: 1.9–914.2) and 8 (IQR: 5.2–10.9) in the [⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DOTANOC, respectively (p = 0.145).

Other distant metastases

Pleural thickening was detected in three patients with higher detection rates and uptake patterns with [⁶⁸Ga]Ga-DOTA.SA.FAPi. Both SULpeak and TBR values for the pleural metastases were notably higher with [⁶⁸Ga]Ga-DOTA.SA.FAPi: 4.3 (IQR: 2–6.6) than that of [⁶⁸Ga]Ga-DOTANOC: 2.1 (IQR: 1.9–5.1); p = 0.0001 (TBR SULpeak: 3.2 [IQR: 1.1–4.2] vs. 1.6 [0.6–2.3]; p = 0.002) (Table 2).

Effect of prior tyrosine kinase inhibitor/mTOR inhibitor treatment on the uptake values

A subgroup analysis to explore the influence of previous tyrosine kinase inhibitor (TKI)/mTOR inhibitor treatment on the SUL values revealed no difference in the radiotracer uptake on both [⁶⁸Ga]Ga-DOTA.SA.FAPi and [¹⁸F]F-FDG PET scans (Table 3).

Table 3.

Comparison of Uptake Values According to Prior Exposure to Other Anticancer Treatment Options

Site of lesion	[⁶⁸Ga]Ga-DOTA.SA.FAPi			[⁶⁸Ga]Ga-DOTANOC
Site of lesion	SULpeak (TKI/mTOR inhibitor treatment group), median (IQR)	SULpeak (TKI/mTOR inhibitor naive group), median (IQR)	p	SULpeak (TKI/mTOR inhibitor treatment group), median (IQR)	SULpeak (TKI/mTOR inhibitor naive group), median (IQR)	p
Primary	6.5 (5.1–7.6)	7.4 (6.3–8.2)	0.546	5.2 (5–5.8)	5.8 (5.2–6.7)	0.687
Lymph node	7.2 (6–8)	8.2 (5.6–12)	0.492	5.9 (5–7)	7 (5.6–8.2)	0.843
Liver metastases	6.3 (5.1–10.9)	6.5 (5.3 11)	0.687	9.6 (4.8–12.3)	7.2 (5.4–8.3)	0.657
Lung metastases	4 (2.7–8.9)	4.2 (2.6–9.9)	0.763	2.2 (1.1–7.4)	2.1 (1–7)	0.632
Pleural thickening	4.3 (2–6.2)	4.1 (2.1–6)	0.695	2.2 (1.1–7.6)	2.1 (1–7.1)	0.952
Bone metastases	7 (3.9–8.2)	7.8 (4.5–7.9)	1.000	2 (1.6–5.8)	1.9 (1.8–3.5)	0.812
Brain metastases	6.6 (3.8–7.1)	6 (3–7.1)	1.000	4 (3–5.1)	4.2 (3–5.3)	0.965

Discussion

Several PET radiotracers such as [¹⁸F][F-FDG, [⁶⁸Ga]Ga-DOTA-labeled SSTR analogs, and [¹⁸F]F-FDOPA have been used for the imaging of MTCs, of which [¹⁸F]F-FDOPA shows the best detection efficiency.^3,9 Currently, according to EANM guidelines, FDOPA should be preferred, but Gallium-68 DOTA-labeled SSTR imaging may be useful in cases of inconclusive diagnosis of FDOPA and to assess the feasibility of peptide receptor targeted radionuclide therapy.³ Despite being a valuable theranostic option, a meta-analysis involving nine studies reported that the tumor detection rate on SSTR-based PET or PET/CT is only 63.5% in recurrent MTC.⁵ Hence, there is an imperative need to introduce and evaluate new radiolabeled theranostic molecular imaging probes, which may be used to accurately diagnose, treat, and follow MTC.

In MTCs, RET proto-oncogene mutations are the primary molecular mechanisms underlying tumorigenesis.² In addition, it is reported that the interaction between the tumor cell and their TME plays the driving force in tumor progression.^10,11 The activation of the stromal cells in the TME, which is also known as the desmoplastic reaction, is a diagnostic pathological marker for tumor progression. CAFs express α-smooth muscle actin (α-SMA), FAP, and extracellular matrix proteins such as tenascin C (Tn-C).

Koperek et al. found that the three fibroblast activation markers α-SMA, FAP, and Tn-C- are highly expressed in the peritumoral and intratumoral stromal compartments of MTCs, and that the expression of FAP and Tn-C correlates with the level of desmoplasia determined by histological analysis.¹² They also found that the presence of desmoplasia is associated with invasion morphology and a higher incidence of LN metastasis in both papillary thyroid cancers (PTCs) and MTCs.^12
–14 CAFs have been identified as having protumor activity and genetic stability,¹⁴ making this stromal cell type an appealing target for novel therapeutic strategies aimed at inhibiting tumor progression by targeting the TME. This study examined FAPi as a potential theranostic probe in MTC.

In our study, 27 MTC patients were evaluated, and unlike [⁶⁸Ga]Ga-DOTANOC PET scan (sensitivity: 73.9%), [⁶⁸Ga]Ga-DOTA.SA.FAPi PET scan detected the majority of the (96%) lesions irrespective of the calcitonin levels. In a study by Ong et al.¹⁵ the results showed better sensitivity of 78% with [¹⁸F]F-FDG when calcitonin levels were >1000 pg/mL, but was of limited use if the calcitonin level is <500 pg/mL.

In this study, although the detection rates with [⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DOTANOC PET imaging were comparable, a significantly high uptake, image contrast, and TBR rate were observed with [⁶⁸Ga]Ga-DOTA.SA.FAPi in the primary and locoregional LN metastases.

Further inspection of 105 liver metastases was detected on CT and [⁶⁸Ga]Ga-DOTA.SA.FAPi, only 49 (46.4%) were positive on [⁶⁸Ga]Ga-DOTANOC PET. In an 18-year-old female patient [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT detected multiple liver metastases that were not identified on [⁶⁸Ga]Ga-DOTANOC PET/CT (Fig. 3). Sahin et al.¹⁶ set diagnostic criteria for identifying 98 liver metastases in 31 patients with gastrointestinal tumors. Among them, 92 lesions were [⁶⁸Ga]Ga-DOTA-FAPI positive, and on the contrary, only 65 were [¹⁸F]F-FDG positive. In agreement with our results, Kuyumcu et al.¹⁷ demonstrated multiple liver metastases on [⁶⁸Ga]Ga-FAPI-04 PET/CT that were not detected on [⁶⁸Ga]Ga-DOTATATE PET/CT in a patient with MTC.

Importantly, we observed that [⁶⁸Ga]Ga-DOTA.SA.FAPi has an exceptionally high detection efficiency and TBR in the liver due to the negligible liver background of SULpeak uptake value as low as 1.3 compared with that of [⁶⁸ Ga]Ga-DOTANOC (average background SULpeak: 5–6) and [¹⁸F]F-FDG (average background SULpeak: 3). This finding reinforces the idea of exploiting [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT for the planning of radiotherapy for liver tumors. Increasing evidence is accumulated on the accuracy of planning radiotherapy with [⁶⁸Ga]Ga-DOTA.SA.FAPi instead of [¹⁸F]F-FDG or [⁶⁸Ga]Ga-DOTANOC in liver and bone metastases attributing to the increased lesion uptake on FAPI PET/CT with a low background of the liver.¹⁸

[⁶⁸Ga]Ga-DOTA.SA.FAPi detected more lung and bone metastases and higher activity than [⁶⁸Ga]Ga-DOTANOC. An 18-year-old female (Fig. 3) showed FAPI uptake in multiple underlying sclerotic bone lesions and liver metastases that were not identified on [⁶⁸Ga]Ga-DOTANOC PET/CT. The possible reason for such high detection efficiency on [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT may be due to the stromal development and abundant stromal components found even at the early stages of tumorigenesis^19,20 in subcentimetric lesions and suggesting an independent role for FAPI PET/CT in improved lesion detection and aiding accurate staging of MTC.

Our preliminary experience on the theranostic approach of [⁶⁸Ga]Ga-DOTA.SA.FAPi guided [¹⁷⁷Lu]Lu-DOTAGA.(SA.FAPi)₂ therapy in a case of aggressive MTC patients highlights a clear survival benefit from FAP inhibitor radionuclide therapy.²¹ More recent evidence from our group reveals clinical benefit from [⁶⁸Ga]Ga/[¹⁷⁷Lu]Lu-DOTAGA.(SA.FAPi)₂ therapy in patients with radioiodine refractory thyroid cancers.²² Our data suggest the promising detection efficiency of [⁶⁸Ga]Ga-DOTA.SA.FAPi is an excellent initial step toward expanding the therapeutic front for MTCs.

Study limitations

Although the present results clearly support imaging with [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT in the restaging of MTC patients, it is appropriate to recognize some potential limitations. The main drawback of diagnostic retrospective studies involves the risk of bias due to deficiencies in the methodology of the study, selection of reference standard or its alternative feasible option, data collection, and execution or interpretation of the test. In this study, the ideal reference standard, histopathology of each lesion was an impractical option, and hence diagnostic quality CT was used as an alternate reference standard. Our study was small and our findings need to be validated in a larger study.

A detailed correlation between the lesion size on CT with the uptake of both radiotracers was not conducted. We did not analyze the association between the molecular signature of the tumor and the tumor uptake of tested radiotracer in this study. Our study results provide a good starting point for discussion and further research. A well-designed prospective diagnostic trial with predefined specific objectives will be quite beneficial in confirming the potential clinical utility of [⁶⁸Ga]Ga-DOTA.SA.FAPi in the accurate detection of lesions, and contribute to understanding disease mechanisms and the development of novel treatments in patients with MTC.

Conclusion

[⁶⁸Ga]Ga-DOTA.SA.FAPi and [⁶⁸Ga]Ga-DOTANOC radiotracers demonstrated similar detection rates in the diagnosis of primary/residual tumors and locoregional LN metastases. [⁶⁸Ga]Ga-DOTA.SA.FAPi exhibited a superior detection efficiency relative to [⁶⁸Ga]Ga-DOTANOC PET imaging when used to detect distant metastatic lesions. We firmly believe that the innovation of therapeutic agents should accompany the development of diagnostic agents.

Footnotes

Institutional Review Board Statement

The study was conducted in accordance with the Declaration of Helsinki, and approved by the Institutional Ethics Committee of All India Institute of Medical Sciences (Ref. No. IECPG: 22/27, April 23, 2020).

Informed Consent

Written informed consent was obtained from all patients to participate in the study.

Authors' Contributions

All authors contributed to the study's conception and design. Material preparation, scan acquisition, data collection, and analysis were performed by S.B., M.P.Y., P.S., S.R., and S.S. The first draft of the article was written by S.B. and M.P.Y. Images were processed and reported by C.B. and M.T. S.A. reviewed the pathology reports. Dr. Ranjit Kumar Sahoo referred patients for scans. F.R., E.S.M., and M.M., our collaborators from the University of Mainz, have synthesized the precursor and finalized the article. All authors commented on previous versions of the article. All authors read and approved the final article.

Disclaimer

This study has not been submitted for review or is not under acceptance for publication in any journal.

Author Disclosure Statement

All the authors included in this article have stated no financial or personal conflict of interest. There is no recent (within the past 5 years), current, or anticipated employment by an organization that may gain or lose financially from the publication of the article.

Funding Information

No funding was received for this article.

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Head-to-Head Comparison of [ 68 Ga]Ga-DOTA.SA.FAPi and [ 68 Ga]Ga-DOTANOC Positron Emission Tomography/Computed Tomography Imaging for the Follow-Up Surveillance of Patients with Medullary Thyroid Cancer

Abstract

Background:

Materials and Methods:

Results:

Conclusion:

Introduction

Materials and Methods

Synthesis of [68Ga]Ga-DOTA.SA.FAPi and quality control

PET/CT acquisition

Data interpretation

Data analysis and processing

Definitions

True-positive lesion

False-positive lesion

True-negative lesion

False-negative lesion

Statistical analysis

Results

Comparison of lesion detection and uptake between [68Ga]Ga-DOTANOC and [68Ga]Ga-DOTA.SA.FAPi PET/CT scans

Primary tumor

Lymph node metastases

Lung metastases

Liver metastases

Bone metastases

Brain metastases

Other distant metastases

Effect of prior tyrosine kinase inhibitor/mTOR inhibitor treatment on the uptake values

Discussion

Study limitations

Conclusion

Footnotes

Institutional Review Board Statement

Informed Consent

Authors' Contributions

Disclaimer

Author Disclosure Statement

Funding Information

References

Synthesis of [⁶⁸Ga]Ga-DOTA.SA.FAPi and quality control

Comparison of lesion detection and uptake between [⁶⁸Ga]Ga-DOTANOC and [⁶⁸Ga]Ga-DOTA.SA.FAPi PET/CT scans