Comparison of ⁶⁸Ga-FAPI and ¹⁸F-DOPA PET/CT in patients with medullary thyroid cancer

Accurate staging is paramount for the effective management of medullary thyroid carcinoma (MTC). Imaging cancer-associated fibroblasts utilizing Gallium-68-labeled fibroblast activation protein inhibitors (FAPI) presents a novel approach for molecular imaging across various cancers, including MTC. This study aimed to compare the diagnostic accuracy of FAP-targeted imaging (⁶⁸Ga-RTX-1363S, abbreviated as ⁶⁸Ga-FAPI) with the established imaging standard, ¹⁸F-DOPA, for the detection of metastases in patients with MTC.

This retrospective study compared ¹⁸F-DOPA- and ⁶⁸Ga-FAPI PET/CT imaging using per-patient and per-lesion analyses in patients with recurrent MTC. Imaging findings were correlated with morphological imaging (CT/MRI/sonography) or histopathology as gold standard. Quantitative assessment included a comparison of standardized uptake values (SUVs) and tumor-to-background ratios (TBRs).

Nine patients (mean age 55 years, range 34-80 years) with 62 lesions were included. Compared to ¹⁸F-DOPA, ⁶⁸Ga-FAPI showed significantly higher lesion detection rates for lymph node metastases (true-positive [TP] rate: 100% [8/8] vs. 50% [4/8]; TBR: 10.9 vs. 6.9), lung metastases (TP rate: 85.7% [12/14] vs. 42.9% [6/14]; TBR: 6.1 vs. 3.6), and liver metastases (TP rate: 100% [22/22] vs. 23% [5/22]; TBR: 24.1 vs. 2.7).

In direct comparison, ⁶⁸Ga-FAPI detected a higher number of metastases with higher TBR values. ⁶⁸Ga-FAPI PET/CT holds promise for improving staging in MTC. However, these findings necessitate confirmation in a larger, prospective study.