A 38-year-old woman with anterior mediastinal Hodgkin lymphoma underwent F-FDG PET/CT, revealing abnormal uptake at the left retrostyloid parapharyngeal space. After chemotherapy, follow-up PET/CT showed regression of previously avid lesions but not the left retropharyngeal mass. Subsequent MRI revealed findings suggestive of neurogenic tumor. Transoral biopsy revealed ganglioneuroma, which is uncommon in the parapharyngeal space, although usually of benign nature. This case therefore highlights the importance of PET/CT in both initial diagnostic screening for tumor development and posttreatment evaluation of ambiguous lesions.