Heterogeneity and Evolution of 68 Ga-Pentixafor PET/CT Signals in Extramedullary Multiple Myeloma.

A 55-year-old man with multiple myeloma (MM) underwent 68 Ga-pentixafor PET/CT. An extramedullary soft-tissue lesion was initially misdiagnosed as a mesenchymal tumor due to low tracer avidity and marked uptake heterogeneity (bone marrow SUVmax 21.8 vs. extramedullary lesion SUVmax 4.1). The patient received standard anti-myeloma therapy for presumed medullary-limited MM. After 4 months, bone marrow disease was controlled, but the isolated extramedullary soft-tissue lesion showed increased 68 Ga-pentixafor uptake and enlarged size. Immunohistochemistry confirmed the extramedullary lesion to be an extramedullary disease (EMD) originating from MM. This case highlights the diagnostic challenges of low-CXCR4-avid extramedullary lesions.