Abstract Pediatric solitary plasmacytomas are extremely rare, with only 14 cases previously reported. None of these reports included PET imaging findings. We present PET/CT and PET/MRI imaging findings in a 15-year-old boy with what was initially thought to be a solitary plasmacytoma, which later developed into multiple myeloma. The initial PET/CT of solitary plasmacytoma, as well as postradiotherapy PET/MRI findings with the new lesions suggestive of multiple myeloma, are presented.