Added Value of Interim FDG PET/CT in Resolving PSA-PSMA Discordance during PSMA-targeted Radioligand Therapy.

Kassas, Mutaz; Artigas, Carlos; Refalo, Nick; Mallia, Andrew; Flamen, Patrick · Clin Nucl Med · 2026

case_report · Level V

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Abstract

PSMA-targeted radioligand therapy (RLT) is effective in metastatic castration-resistant prostate cancer, but interpretation is challenging when PSA rises despite apparent response on post-therapy 177 Lu-PSMA SPECT/CT. We present 2 patients with this PSA-PSMA discordance after 2 cycles: 177 Lu-PSMA post-therapy SPECT/CT showed a significant reduction in PSMA-derived tumor volume, yet PSA levels progressed. 18 F-FDG PET/CT performed after 2 cycles clarified disease status by revealing new hypermetabolic skeletal and visceral lesions, consistent with progression. PSMA-targeted RLT was discontinued early in both cases. 18 F-FDG PET/CT may resolve discordant PSA and PSMA findings and help prevent futile continuation of ineffective PSMA‑targeted RLT.

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