Scalp Soft Tissue FDG Avidity Mimicking Extracranial Seeding in Glioblastoma: A Diagnostic Pitfall on ¹⁸ F-FDG PET/CT

Extracranial metastasis of glioblastoma (GBM; WHO Grade IV astrocytoma) is exceedingly rare, and new scalp lesions in post-craniotomy patients undergoing 18 F-fluorodeoxyglucose (FDG) PET/CT surveillance carry significant diagnostic ambiguity. We report a case of 23-year-old woman with a WHO Grade IV astrocytoma who, following surgical resection and concurrent chemoradiotherapy (Stupp protocol) presented with scalp swelling. The patient underwent 18 F-FDG PET/CT that demonstrated a focal hypermetabolic soft tissue nodule (SUVmax19.2) along the operative scar, raising a strong suspicion for surgical tract seeding. Fine-needle aspiration cytology (FNAC) of the scalp nodule revealed a benign histiocytic reaction with no evidence of malignancy, confirming false-positive FDG uptake. This case illustrates a critical diagnostic pitfall: post-surgical inflammatory processes—including granulation tissue, suture granulomas, and histiocytic infiltrates—can produce intensely FDG-avid scalp nodules that closely mimic extracranial GBM seeding. Integration of CT morphology, clinical context, and histopathological confirmation is indispensable before attributing such lesions to metastatic disease.