Diagnostic Tissue Re-Evaluation Supporting the Observation of a 11C-Methionine PET-Negative Residual Paraskeletal Mass in Multiple Myeloma.

A 52-year-old man with Bence Jones protein λ-type multiple myeloma developed a large sacral bone-associated, paraskeletal mass. After radiotherapy and idecabtagene vicleucel, serum and urine immunofixation tests were negative, marrow minimal residual disease assessment was negative, and post-treatment 11C-methionine positron emission tomography/computed tomography showed no abnormal uptake in the residual lesion, although computed tomography showed a persistent mass. A re-biopsy was not clinically justified in this case. A re-evaluation of the diagnostic tissue revealed plasma cell-rich areas and acellular collagen-rich stroma with extracellular matrix-associated proteins and λ light chain-derived peptides. Observation was selected, and the mass gradually decreased without any clinical relapse.