Multiple Myeloma (MM) is a hematologic malignancy characterized by the clonal proliferation of plasma cells, accounting for approximately 10–15% of hematopoietic neoplasms. The main clinical manifestations include renal failure, bone lesions, anemia, hypercalcemia, and leukopenia. Plasmacytoma is a localized plasma cell lesion that may or may not be associated with MM and can affect the mandible with some frequency. Conversely, plasma cell infiltration in the oral mucosa is less common and generally associated with disease activity. In this context, the diagnosis of oral cavity lesions in MM patients may serve as a tool for monitoring disease progression and therapeutic response to chemotherapy.

To report a case in which the diagnosis of MM infiltration in the oral cavity determined treatment failure and disease progression.

A 58-year-old male patient with a history of heart failure was diagnosed with Kappa MM, DS IIA/ISS I, presenting with femoral fracture and hypercalcemia. He was initially treated with two cycles of VCD (Bortezomib + Cyclophosphamide + Dexamethasone) but showed disease progression (Kappa light chain increase >25%), prompting a change to VTD (Bortezomib + Thalidomide + Dexamethasone). After two cycles, he presented minimal response with partial Kappa reduction but new-onset proteinuria. During a routine consultation at the Dentistry and Oral Medicine service, a painless swelling with obliteration of the left mandibular vestibule was observed, with rubbery consistency, ill-defined margins, and no surface changes. Panoramic radiography revealed multiple radiolucent areas in the mandibular body, showing the classic MM bone pattern. An incisional biopsy confirmed plasma cell neoplasia, with marked nuclear and cellular pleomorphism, bi- and multinucleated cells, immunohistochemical positivity for CD138 and Kappa light chain, and a Ki-67 index of 20%, confirming disease progression despite chemotherapy. The patient developed renal and respiratory complications, culminating in multiple organ failure and death, precluding further MM treatment.

This case highlights the importance of multidisciplinary follow-up by a trained dental team to identify and diagnose oral alterations that may contribute to disease staging and therapeutic monitoring in patients with Multiple Myeloma.