Increased 18F-FDG Avidity on PET/CT During Treatment for Rhabdomyosarcoma Confirmed to Be Mature Rhabdomyoblasts

Background: Fluorine-18 fluorodeoxyglucose positron emission tomography ( 18 F-FDG-PET) is useful for staging rhabdomyosarcoma. Using this modality as a predictor of outcome and for disease surveillance in rhabdomyosarcoma is conflicting. Observations: Case report of an adolescent being treated for metastatic fusion-positive rhabdomyosarcoma who had a new 18 F-FDG-PET avid focus in the radiation field of the primary site with increasing standard uptake value during systemic chemotherapy and primary tumor local therapy that was worrisome for disease progression. Conclusions: This lesion was resected and confirmed as terminally cytodifferentiated (mature) rhabdomyoblasts on histopathology. He remains in remission 26 months following resection and 20 months following treatment.