Multiple myeloma (MM) is a malignant neoplasm characterized by clonal proliferation of plasma cells in the bone marrow, generally associated with monoclonal immunoglobulin production and clinical manifestations such as lytic bone lesions, anemia, hypercalcemia, and renal failure. Plasmacytoma is an extramedullary proliferation of monoclonal neoplastic plasma cells that may present as a solitary bone lesion or in soft tissues.

To present a case of oral plasmacytoma as an isolated manifestation during post-transplant follow-up in a patient with multiple myeloma treated with autologous hematopoietic stem cell transplantation.

A 39-year-old male patient was diagnosed with IgA Kappa MM, DS IIIA, ISS 1, in October 2022. Myelogram revealed 24% plasma cells, some binucleated, along with multiple lytic bone lesions, anemia, and hypercalcemia, fulfilling diagnostic criteria. Treatment was initiated with two pulses of dexamethasone (40 mg/day), followed by eight cycles of chemotherapy with the VTD protocol (bortezomib, thalidomide, and dexamethasone), achieving complete response after the final cycle in October 2023. In March 2024, the patient had disease progression in the form of a plasmacytoma at T8, without systemic relapse. Local radiotherapy (20 Gy in 5 fractions) was administered, with complete response confirmed on PET- CT in March 2024. In April 2024, the patient underwent autologous bone marrow transplantation. During routine dental follow-up, a sessile, nodular lesion approximately 1 cm in size, with a regular surface, soft consistency, was erythematous, hypervascularized with bleeding upon manipulation, was noted in the gingiva distal to tooth 17. Given the clinical history, an incisional biopsy was performed. Histopathological examination revealed a dense infiltrate of atypical plasma cells positive for CD138, CD56, and MUM1. Light chain analysis demonstrated kappa light chain monoclonality, confirming the diagnosis of extramedullary plasmacytoma of the oral mucosa. This manifestation occurred in the absence of laboratory signs of systemic relapse. The patient was referred back to the hematology team for further evaluation. Conclusion: This case underscores the critical role of regular dental surveillance in patients with multiple myeloma, even after complete remission and hematopoietic stem cell transplantation. Extramedullary manifestations—such as oral plasmacytoma—may arise in the absence of systemic disease activity. Early detection through timely diagnosis and referral is essential, as it can identify initial signs of progression or relapse and has a direct impact on patient prognosis.