Evaluation of fibroinflammatory activity in thyroid eye disease using [18F]AlF-NOTA-FAPI-04 PET/CT: A prospective study

Abstract

Purpose: This study evaluates the value of [¹⁸F]AlF-NOTA-FAPI-04PET/CT in assessing the disease activity of thyroid eye disease (TED).

Methods: Twenty-three TED patients and ten healthy controls were prospectively recruited and underwent [¹⁸F]AlF-NOTA-FAPI-04 PET/CT scans. TED eyes were classified by Clinical Activity Score (CAS) as active (CAS +) or inactive (CAS-). PET analysis included qualitative and quantitative evaluations. Qualitative analysis categorized TED eyes as positive (FAPI +) or negative (FAPI-), while quantitative analysis including SUVmax and target-to-backgroud ratio (TBR) of each extraocular muscle (EOM). Pathological assessment of 16 EOM samples from 15 TED eyes was used as a gold standard to evaluate the diagnostic accuracy of CAS-driven and FAPI-driven assessments.

Results: Eye-based analysis included 42 TED eyes and 20 control eyes. SUVmax did not differ significantly between CAS-/FAPI + (n = 8) and CAS + /FAPI + (n = 17) eyes; though both groups had significantly higher SUVmax than CAS-/FAPI- (n = 16), CAS + /FAPI- (n = 1), and control eyes (n = 20) (all P < .05). Diffuse high FAPI uptake with excellent TBR was observed in pathologically active EOMs and had higher SUVmax and TBR than inactive EOMs (SUVmax: 9.57 ± 2.79 vs. 4.04 ± 1.02, TBR: 2.99 ± 1.00 vs. 1.00 ± 0.35, both P < .001). An SUVmax cutoff value of 5.95 yielded an area under the receiver operating characteristic curve of 0.98 for predicting active disease. Sensitivity and specificity of CAS-driven and FAPI-driven assessments for TED activity were 72.7% and 100%, 100% and 100%, respectively. SUVmax positively correlated with pathological fibroinflammatory score (r = 0.68, P = .004).

Conclusion: The diffuse high FAPI uptake with excellent TBR in the EOMs could illustrate the activity of TED, positively correlated with pathological fibroinflammatory scores. These findings suggest that [¹⁸F]AlF-NOTA-FAPI-04 PET/CT may serve as a promising reliable non-invasive approach for assessing disease activity in TED, potentially superior to the CAS.

Keywords: Clinical activity score; Extraocular muscles; Fibroblast activation protein inhibitor; Thyroid eye disease.

Fig. 1

Flowchart of participant enrollment and study profile. CAS = Clinical Activity Score, FAPI = fibroblast activation protein inhibitor, TED = thyroid eye disease

Fig. 2

Maximum-intensity-projection (MIP) images of [¹⁸F]AlF-NOTA-FAPI-04 PET/CT from all participants and heatmap for eye-based analysis. A MIP images of FAPI-positive (FAPI +) patients, showing partial or all extraocular muscles (EOMs) with high FAPI uptake. Patients were assigned as #1-#15. Clinical Activity Score (CAS)-driven assessment results were assigned to each eye. B MIP images of FAPI-negative (FAPI-) patients, showing all EOMs with mild to moderate FAPI uptake. Patients were assigned as #16-#23. CAS-driven assessment results were assigned to each eye. C MIP images of control participants, assigned as Control #1-#10, demonstrating faint to mild FAPI uptake in the EOMs of control eyes. D Heatmap illustrating the CAS-driven and FAPI-driven assessment results, along with the maximum standardized uptake value (SUVmax) for each eye (L = left eye, R = right eye). NA = not applicable, postoperative eyes were excluded

Fig. 3

Quantitative analyses across all groups. A Patient-based analysis. B Eye-based analysis. The differences in maximum standardized uptake value (SUVmax) and target-to-background ratio (TBR) of extraocular muscles were significant among all groups in patient-based and eye-based analyses (all P < .001). CAS = Clinical Activity Score, CAS- = patients or eyes with inactive CAS scores. CAS +  = patients or eyes with active CAS scores. FAPI = fibroblast activation protein inhibitor, FAPI- = patients or eyes classified as negative on [¹⁸F]AlF-NOTA-FAPI-04 PET/CT. FAPI +  = patients or eyes classified as positive on [¹⁸F]AlF-NOTA-FAPI-04 PET/CT. All statistically significant differences and important non-statistically significant differences among multiple comparisons were summarized in the figure. Statistical significance is indicated as follows: *P < .05, **P < .01, ***P < .001, and ns = no significant difference

Fig. 4

Images of patients #22, #3, and #5 with consistent Clinical Activity Score (CAS)-driven and Fibroblast Activation Protein Inhibitor (FAPI)-driven assessment results. (A-C) Left panel: external photograph. Middle panel: [¹⁸F]AlF-NOTA-FAPI-04 PET/CT images. Right panel: representative histopathological figures (low-power microscopic examination with scale bar of 200 μm, high-power microscopic examination with scale bar of 50 μm) using Hematoxylin–eosin (H&E) staining (left), Masson trichrome staining (middle), and fibroblast activation protein (FAP) immunochemistry staining (right) of the extraocular muscles (EOMs). A Patient #22, mild to moderate FAPI uptake in EOMs was observed. Histopathological figures from the left eye inferior rectus muscle (arrows, SUVmax = 4.3). IG = I0-I1, FG = F0-F1. B Patient #3, high FAPI uptake in EOMs was observed. Histopathological figures from the right eye medial rectus muscle (arrows, SUVmax = 6.4) showed high expression of FAP. IG = I3, FG = F2. C Patient #5, intense FAPI uptake in EOMs was observed. Histopathological figures from the left eye inferior rectus muscle (arrows, SUVmax = 13.8) showed high expression of FAP. IG = I3, FG = F3. IG = Inflammation grade, FG = fibrosis grade

Fig. 5

Images of patients #10 and #6 with inconsistent Clinical Activity Score (CAS)-driven and Fibroblast Activation Protein Inhibitor (FAPI)-driven assessment results. (A-C) Left panel: external photograph. Middle panel: [¹⁸F]AlF-NOTA-FAPI-04 PET/CT images. Right panel: representative histopathological figures (low-power microscopic examination with scale bar of 200 μm, high-power microscopic examination with scale bar of 50 μm) using Hematoxylin–eosin (H&E) staining (left), Masson trichrome staining (middle), and fibroblast activation protein (FAP) immunochemistry staining (right) of the extraocular muscles (EOMs). (A) Patient #10, intense FAPI uptake in EOMs of right eye was observed. Histopathological figures from the right eye medial rectus muscle (arrows, SUVmax = 12.2) showed high expression of FAP. IG = I4, FG = F4. (B) Patient #6, intense FAPI uptake in EOMs was observed. Histopathological figures from the left eye medial rectus muscle (arrows, SUVmax = 10.9) showed high expression of FAP. IG = I3, FG = F3. IG = Inflammation grade, FG = fibrosis grade