Direct bone marrow assessment by microscopic evaluation of bone marrow aspirates and trephine biopsies has been the gold standard in bone marrow staging of children and adolescents diagnosed with cancer. Recently, the need to perform bone marrow biopsies in patients with low-risk rhabdomyosarcoma without local invasion or evidence for lymph node/lung metastases has been called into question, as retrospective analyses demonstrated 0% bone marrow metastases in such patients [7]. Another retrospective assessment revealed detection of bone marrow metastases with 92% sensitivity/94% specificity by 2-[18F]FDG-PET/CTs compared to direct bone marrow sampling [8]. Yet, as four of 114 patients had PET/biopsy + bone marrow disease, it was concluded that bone marrow biopsies could not be omitted from the initial staging of patients diagnosed with rhabdomyosarcoma [8]. Alternatively, whole body MRI may allow for non-invasive detection of bone marrow metastases without radiation exposure, but with long imaging times.

The case reported here illustrates detection of bone marrow metastases in a 6-year-old boy with parameningeal rhabdomyosarcoma by combined assessment of 2-[18F]FDG-PET/CT and MRI. As none of the 2-[18F]FDG-avid bone marrow sites was biopsied directly, the lack of cytological validation of bone marrow disease represents a limitation of this report. Albeit extremely unlikely, we cannot completely rule out other causes of increased 2-[18F]FDG uptake such as chronic non-bacterial osteomyelitis or Langerhans cell histiocytosis. When using 2-[18F]FDG PET/CT for bone marrow staging, its limitations should be kept in mind: 2-[18F]FDG PET/CT imaging is associated with exposure to ionizing radiation, requires anesthesia in young children, does not depict 2-[18F]FDG-negative or very small lesions, and may visualize reactive changes in bone marrow activity (e.g., following chemotherapy), which must not be mistaken for tumor spread. Also, distinguishing bone from bone marrow metastases represents a challenge and requires consideration of signs of cortical/trabecular destruction, presence of soft tissue masses, and abnormal bone marrow signal depicted by MRI.

Arguably, these limitations are outweighed by major advantages associated with bone marrow staging by 2-[18F]FDG PET/CT: above all, the stress, pain, and need for some form of anesthesia—typically associated with direct bone marrow sampling by bone marrow aspirates/ trephine biopsies—can be avoided. Also, bone marrow metastases may be detected more reliably. Bone marrow infiltration by solid tumor or lymphoma cells is often patchy, and bone marrow staging based on bone marrow aspirates/trephine biopsies (typically obtained from the right and left posterior iliac crests) comes with a degree of sampling error. 2-[18F]FDG PET/CT allows depiction of the entire trunk and limbs and may identify sites of bone marrow disease that are otherwise not detectable. This may change disease stage and, consequently, intensity of treatment in some patients. Widespread use of 2-[18F]FDG PET/CT in rhabdomyosarcoma staging may ultimately require redefinition of bone marrow disease and its impact on risk stratification.