Cardiac plasmacytoma with extensive infiltration: multimodal diagnosis and therapeutic challenges: a case report

Background: Cardiac involvement in multiple myeloma (MM) in the form of cardiac plasmacytoma is exceedingly rare and carries a poor prognosis. Case summary: We describe the case of a 59-year-old man with serological relapse of MM in whom a hypermetabolic cardiac mass was detected on positron emission tomography-computed tomography. Cardiac magnetic resonance imaging revealed extensive atrial infiltration with imaging features suggestive of malignancy. Histopathological examination confirmed the diagnosis of cardiac plasmacytoma. The patient was started on systemic anti-CD38 based therapy, which led to a rapid and substantial reduction in tumour burden. Persistent metabolic activity prompted the addition of localized radiotherapy and treatment escalation with a novel bispecific antibody. Discussion: This case contributes to the limited literature on cardiac plasmacytoma. The role of multimodality imaging for mass characterization is highlighted. Yet, imaging characteristics alone may be indistinguishable from those of primary cardiac malignancies, making tissue diagnosis crucial. Despite advances in systemic therapies that may induce marked tumour responses, long-term prognosis remains guarded. Notably, this case represents one of the longest reported survivals among patients with cardiac involvement.