Radiopharmaceuticals in Malignant Melanoma: A Comprehensive Review of Diagnostic, Therapeutic, and Immune-Related Applications by PET/CT, SPECT/CT, and PET/MRI

Background: Malignant melanoma remains an oncological challenge, with advanced-stage five-year survival rates under 20%. Precise molecular imaging has become indispensable for accurate staging, selection of targeted or immunotherapies, treatment response assessment, and early detection of immune-related adverse events. This review examines the roles of PET/CT, PET/MRI, and SPECT/CT radiopharmaceuticals in melanoma management and highlights novel tracers and theranostic strategies poised to enhance precision nuclear medicine in this disease. Methods: We performed a review of English-language literature from January 2000 through June 2025, querying PubMed, Scopus, and clinical-trial registries for original research articles, meta-analyses, clinical guidelines, and illustrative case reports. Eligible studies investigated PET/CT, PET/MRI, or SPECT/CT applications in melanoma diagnosis, nodal and distant staging, therapy monitoring, irAE (immune-related adverse events) detection, and the development of emerging radiotracers or theranostic radiopharmaceutical pairs. Results:18F-FDG PET/CT demonstrated a high detection rate for distant metastases, outperforming conventional CT and MRI in advanced disease, despite limited resolution for infracentimetric nodal deposits. PET/MRI offers comparable diagnostic accuracy with superior soft-tissue contrast and improved brain lesion detection, while SPECT/CT enhanced sentinel lymph node localization prior to surgical biopsy. Also, FDG PET/CT identified visceral irAEs with great sensitivities, revealing asymptomatic toxicities in up to one-third of patients. Emerging radiotracers targeting melanin, fibroblast activation protein, PD-1 (programmed cell death protein 1)/PD-L1 (programmed cell death-ligand 1), and CD8+ T cells have demonstrated enhanced tumor specificity and are on their way to forming novel theranostic pairs. Conclusions: While 18F-FDG PET/CT remains the cornerstone of melanoma imaging, complementary advantages of PET/MRI and SPECT/CT imaging refine melanoma management. The advent of highly specific radiotracers and integrated theranostic approaches heralds a new era of tailored nuclear-medicine strategies, promising improved patient stratification, therapy guidance, and clinical outcomes in melanoma.