A Diagnostic Pitfall on 18F-FDG PET/CT: Transient Vertebral FDG Uptake During Immunotherapy

An 80-year-old man with poorly differentiated squamous cell carcinoma underwent 18 F-FDG PET/CT for initial staging and treatment response assessment following pembrolizumab therapy. Follow-up PET/CT demonstrated a new FDG-avid focus within the T3 vertebral body without a corresponding CT abnormality. Subsequent contrast-enhanced MRI revealed no structural correlate at the site of uptake. A repeat PET/CT performed 3 months later showed complete resolution of the vertebral lesion without additional treatment. The imaging findings favored transient immune-related inflammatory uptake or a pseudoprogression-like phenomenon rather than true osseous metastasis.