A 38-year-old woman was referred for staging of an alveolar rhabdomyosarcoma revealed by a breast metastasis with unknown primary. Whole-body F-FDG PET/CT localized the primary tumor as an asymptomatic soft tissue lesion of the right foot with a significant uptake. PET/CT detected hepatic and pancreatic metastases and also showed a multifocal bone marrow involvement, which was not detected by the contrast-enhanced CT. These results led to a significant change in treatment management as the patient received local radiotherapy of the primitive lesion and polychemotherapy.