A Case of Maxillary Sinus Plasmacytoma With the Rare Manifestation of Oroantral Fistula: A Case Report

Background Extramedullary plasmacytoma (EMP) is an uncommon neoplastic proliferation of monoclonal plasma cells, most frequently arising within the upper respiratory tract. Involvement of the maxillary sinus is rare, and presentation as a chronic oroantral fistula (OAF) is exceptionally unusual, often complicating timely diagnosis and management. Case Presentation A 63‐year‐old female presented with a 9‐month history of unintentional communication between the oral cavity and nasal passages, resulting in the regurgitation of food and liquids into the nose. Clinical examination identified a persistent OAF in the left maxillary vestibule without a history of trauma, dental extraction, or sinus pathology. Initial radiographic assessment and subsequent advanced imaging, including cone‐beam computed tomography (CBCT), magnetic resonance imaging (MRI), and positron emission tomography/computed tomography (PET/CT), revealed a solitary soft‐tissue mass occupying the left maxillary sinus with erosion of the anterior sinus wall. Histopathological and immunohistochemical (IHC) evaluation confirmed the diagnosis of EMP. Systemic workup, including serum protein electrophoresis and bone marrow biopsy, excluded multiple myeloma (MM). Management and Outcome Surgical treatment involved complete excision and curettage of the sinus lesion with cauterization of the fistulous tract, followed by successful OAF closure using a pterygomandibular buccal fat pad flap. Adjuvant external‐beam radiotherapy was administered at a total dose of 50 Gy in 25 fractions. At the 1‐year follow‐up, the patient demonstrated complete healing of the oroantral communication, full resolution of nasal reflux symptoms, and no clinical or metabolic evidence of local recurrence or systemic progression on PET/CT. Conclusion This case highlights an exceptionally rare presentation of maxillary sinus EMP manifesting primarily as a chronic OAF. Accurate diagnosis supported by advanced imaging, thorough systemic evaluation, and coordinated surgical–oncologic management contributed to a favorable outcome. Clinicians should consider EMP in the differential diagnosis of nonhealing OAFs without an apparent odontogenic cause.