Seiji Kakiuchi, Chiharu Mishima, Kanae Yamakawa, Naru Tomigaki, Akimasa Sakamoto, Shutaro Fujioka, Isamu Harima, Hiroaki Akiyama, Ryotaro Niwa, Ikumi Takagi, Yoko Kozuki, Yoshiharu Miyata, Nobuko Iwata, Kyoko Yoshihara, Satoshi Yoshihara, Kyoko Otani
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.