Alveolar rhabdomyosarcoma (RMS) accounts for 20% to 30% of childhood RMS and is associated with a prognosis worse than embryonal RMS. Disseminated RMS can present with extensive bone marrow involvement. Assessing the extent of the tumor is critical, because therapy and prognosis depend on the degree to which the mass has spread beyond the primary site. The value of F-18 FDG PET in patients with RMS has been reported in some series but none specifically involving bone marrow. Children have a highly cellular hematopoietic bone marrow and differentiation of a highly cellular marrow from neoplastic infiltration may be difficult. Various other conditions associated with diffuse FDG uptake in the bone marrow include marrow hyperplasia resulting from hemolytic/iron-deficiency/blood-loss anemia, after chemotherapy with granulocyte/macrophage colony-stimulating factor and recombinant erythropoietin treatment, leukemia, non-Hodgkin lymphoma, and myelodysplasia. It is therefore important to consider the above differential diagnoses in mind when evaluating cases of unexpected marrow uptake in F-18 FDG PET studies. We report here a case of RMS with diffuse bone marrow involvement detected on F-18 FDG PET wherein FDG PET was useful in determining the true extent (primary and metastases) of RMS before definitive therapy and the valuable adjunct role it has with structural imaging in management.

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