Unsuspected Spondylolysis on FDG PET/CT in a Pediatric Patient With Osteosarcoma

Lumbar spondylolysis can be asymptomatic but is a common cause of low back pain in adolescent athletes. We report a rare case of focal increased FDG uptake at the left pars interarticularis of L5 vertebra, along with an associated transverse fracture line on 18F-FDG PET/CT, in a 15-year-old boy with newly diagnosed osteosarcoma. This led to the incidental diagnosis of L5 spondylolysis. Attention to CT findings and history-taking is prerequisite to avoid misinterpretation of the FDG PET/CT in this clinical setting.