A 7-month-old boy with malignant rhabdoid tumor of the right lateral neck, status post resection and chemotherapy, underwent FDG PET/CT for restaging. The images showed diffuse increased activity in the spleen and in the bone marrow of the appendicular bones and the spine, which is related to hematopoietin administered after chemotherapy. The images also revealed intense activity in the region of sphenoid bone, which is not a common region to have elevated FDG activity. The subsequent MRI scan showed that this activity was due to not-yet-converted red marrow in the sphenoid bone in this pediatric patient.