Mandibular Plasmacytoma as the Initial Manifestation of Non-secretory Multiple Myeloma: A Rare Diagnostic Challenge

Plasma cell neoplasms involving the jaws are rare, with mandibular involvement representing an uncommon initial manifestation of systemic multiple myeloma (MM). Their nonspecific clinical and radiographic presentation often mimics more common odontogenic and inflammatory conditions, making early diagnosis challenging. We report the case of an 81-year-old man who presented with pain and swelling over the left posterior mandible of two months' duration. Clinical examination revealed diffuse swelling involving the left mandibular body and angle. Orthopantomography (OPG) demonstrated a well-defined unilocular radiolucent lesion extending from the left mandibular second premolar to the retromolar region with associated cortical expansion. The histopathological examination of an incisional biopsy revealed diffuse sheets of atypical plasma cells. Initial immunohistochemistry (IHC) demonstrated CD56 positivity, and subsequent evaluation showed diffuse membranous CD138 positivity with kappa light-chain restriction and lambda negativity, confirming a monoclonal (M) plasma cell neoplasm. To differentiate a solitary bone plasmacytoma (SBP) from systemic disease, whole-body 18F-fluorodeoxyglucose positron emission tomography-computed tomography (18F-FDG PET-CT) was performed. Imaging demonstrated a hypermetabolic destructive soft tissue mass involving the left hemimandible measuring 4.6 × 3.9 cm (maximum standardized uptake value {SUVmax}: 16.5), along with multiple FDG-avid osteolytic lesions involving the frontal calvarium, left fourth rib, left iliac bone, and cervical, thoracic, and lumbar vertebrae. Bone marrow biopsy revealed marked plasma cell infiltration, and the correlation of the clinical, radiological, histopathological, immunophenotypic, and marrow findings established the diagnosis of multiple myeloma presenting as a mandibular plasmacytoma. The patient subsequently received palliative external beam radiotherapy (20 Gy in five fractions) to the mandibular lesion and was referred for systemic hematologic management. This case underscores the importance of considering plasma cell neoplasms in the differential diagnosis of destructive mandibular lesions and highlights the indispensable role of comprehensive diagnostic evaluation, including immunohistochemistry, whole-body PET-CT, and bone marrow examination, in distinguishing localized plasmacytoma from multiple myeloma. Early recognition and multidisciplinary management are essential to ensure timely diagnosis, accurate staging, and appropriate treatment, ultimately improving patient outcomes.