<p>Solitary plasmacytoma is a rare form of plasma cell dyscrasia that presents as a single mass of clonal plasma cells. A 70-year-old man presented with a left frontal bone tumor. The patient underwent surgical resection of the tumor and was diagnosed with solitary bone plasmacytoma. Three years later, he developed a tumor in the pericardial cavity, and its biopsy revealed the recurrence of plasmacytoma. Bone marrow examination, including measurable residual disease testing at a sensitivity of 10^–5, showed no signs of clonal plasma cells. He received BLd (bortezomib, lenalidomide, and dexamethasone) and achieved complete response. After nine cycles of BLd, he received maintenance therapy with ixazomib for 1 year, which was switched to lenalidomide maintenance due to nausea and appetite loss with ixazomib. The patient has remained in complete response for three and half years after the recurrence in the pericardial cavity. Clinically, the report highlights the importance of considering the possibility of plasmacytoma in the cardiovascular structures in patients with a history of plasma cell disorders. Biologically, it suggests that the intrinsic characteristics of clonal plasma cells and/or microenvironment of the bone marrow are associated with the localization of the abnormal plasma cells outside the bone marrow.</p>