Poorly Differentiated Neuroendocrine Tumor With 18F-Fluciclovine Uptake in a Patient With Metastatic Castrate-Resistant Prostate Cancer.

18F-Fluciclovine is an amino acid-based radiopharmaceutical used primarily for PET imaging of patients with biochemical recurrence of prostate cancer. We report a case of a 66-year-old man with recently diagnosed metastatic castrate-resistant prostate cancer and a left supraclavicular lymph node with incidental radiotracer uptake on 18F-fluciclovine PET/CT. Left neck core needle biopsy confirmed high-grade, poorly differentiated carcinoma with neuroendocrine features positive for synaptophysin and chromogranin, and negative for prostate markers.