Atypical Skull Base Metastasis from Pediatric Extremity Chondroblastic Osteosarcoma Detected by Surveillance 18F-FDG PET/CT: A Case Report with Focused Literature Review

A 15-year-old girl who had previously undergone surgery and chemotherapy for non-metastatic chondroblastic osteosarcoma of the left fibula was evaluated with 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) during routine follow-up while in clinical remission. Imaging demonstrated a 36 × 17 mm hypermetabolic lesion in the right occipital condyle maximum standard uptake value [(SUVmax): 16.5], with adjacent bone and soft-tissue extension, together with a concurrent 18F-FDG-avid pulmonary nodule (SUVmax: 3.4). Shortly after imaging, the patient developed mild neck discomfort and right-sided tongue deviation, suggesting hypoglossal nerve involvement. Biopsy of the skull base lesion and histopathological evaluation of the pulmonary nodule confirmed metastatic chondroblastic osteosarcoma with concordant morphology and immunophenotype. The patient underwent biopsy of the occipital lesion, surgical excision, radiotherapy, and wedge resection of the pulmonary metastasis. Skull base involvement from osteosarcoma arising in the extremities is exceptional and is particularly rare in pediatric patients. This case highlights the complementary role of whole-body 18F-FDG PET/CT in detecting atypical metastatic sites that may not be routinely assessed by conventional surveillance strategies. In addition, the focused literature review underscores the exceptional rarity of skull base involvement in pediatric chondroblastic osteosarcoma and supports the need for heightened awareness of uncommon metastatic patterns.