Red bone marrow can become stimulated and metabolically active as a rebound phenomenon after chemotherapy, in response to severe or chronic hemorrhage, or by bone marrow stimulants used in oncology patients (e.g., granulocyte-colony stimulating factor, erythropoietin or interleukin-3) [1–5]. In such settings, bone marrow uptake of 18F-FDG at PET can be markedly increased and simulate diffuse metastatic disease (Figure 100.1).
An incorrect diagnosis of metastases due to increased FDG uptake at PET by red marrow conversion could lead to unnecessary additional treatment or inappropriate changes in management . Conversely, it is also possible that this appearance could mask true bone metastases [5, 6].
Typical clinical scenario
Increased bone marrow activity at PET has been reported primarily in cancer patients treated with colony stimulating factors.
The main differential consideration for widespread bone marrow uptake of FDG by converted red marrow is diffuse medullary metastases (Figure 100.2). In practice, medullary metastases are usually focal while red marrow conversion is usually diffuse, but this rule is not absolute – medullary metastases are occasionally diffuse  and red marrow conversion is occasionally focal [8, 9]. The evolution of these abnormalities over time may allow accurate differentiation. If critical to pending management decisions, biopsy may be required.