Purpose

Intracytoplasmic melanin pigment is a characteristic of clear cell sarcoma (CCS), which is a particularly deadly type of soft-tissue sarcoma. [¹⁸F]-N-(2-(diethylamino)ethyl)-5-(2-(2-(2-fluoroethoxy)ethoxy)ethoxy)picolinamide ([¹⁸F]-PFPN) is a positron emission tomography (PET) probe characterized by high melanin affinity. Therefore, this study aimed to investigate the feasibility of melanin-targeted [¹⁸F]-PFPN PET in patients with CCS.

Methods

This prospective single-centre study recruited patients with pathologically confirmed CCS. [¹⁸F]-FDG PET/computed tomography and [¹⁸F]-PFPN PET/magnetic resonance imaging scans were performed within 1 week of each other. The lesion numbers and [¹⁸F]-FDG and [¹⁸F]-PFPN PET parameters (maximum standardized uptake value [SUVmax], mean standardized uptake value [SUVmean], metabolic/melanotic tumour volume [MTV/MLTV], and total lesion glycolysis/melanin [TLG/TLM]) were collected.

Results

Three patients with CCS were recruited and received PET imaging. A total of 56 lesions were detected on [¹⁸F]-PFPN PET, including primary tumour and distant metastases. Identical lesions were not detected on [¹⁸F]-PFPN and [¹⁸F]-FDG PET. Twelve lesions (12/39, 30.77%) on [¹⁸F]-FDG imaging were missed on [¹⁸F]-PFPN, and 20 lesions (20/47, 42.55%) on [¹⁸F]-PFPN imaging were missed on [¹⁸F]-FDG. In quantitative analysis, the [¹⁸F]-FDG SUVmean (4.60 ± 3.24) was higher than the [¹⁸F]-PFPN SUVmean (3.0 ± 2.63) in all lesions (P = 0.01). No significant correlations were found between the SUVmax, SUVmean, MLTV/MTV, and TLM/TLG values of [¹⁸F]-PFPN and [¹⁸F]-FDG (P > 0.05).

Conclusion

Melanin-targeted [¹⁸F]-PFPN PET imaging is feasible for the diagnosis of CCS. Different imaging features were displayed on [¹⁸F]-PFPN and [¹⁸F]-FDG PET imaging, demonstrating the complementary role of the tracers. Combined use of the two imaging modalities would be preferred in patients with CCS.

Clinical Trial Registration

NCT05963035.