Volumetric Modulated Arc Therapy for a Rare Solitary Extramedullary Plasmacytoma of the Nasopharynx: Clinical and Technical Insights

Solitary extramedullary plasmacytoma (SEP) of the nasopharynx is an exceptional entity within upper aerodigestive tract plasma cell neoplasms. We report the case of a 38-year-old previously healthy male who was referred with a two-year history of progressive bilateral nasal obstruction without other regional or systemic symptoms. Nasopharyngeal magnetic resonance imaging (MRI) revealed a well-circumscribed, 38-mm expansile soft-tissue mass localized to the nasopharynx with intact pharyngobasilar fascia and no evidence of cervical lymphadenopathy. Endoscopic biopsy demonstrated a dense plasma cell proliferation with kappa light-chain restriction on immunohistochemistry (positive for CD138, CD56, and MUM1; negative for AE1/AE3, CD68, CD20, and CD3), establishing a diagnosis of nasopharyngeal plasmacytic proliferation with kappa monotypia. Staging with whole-body 18F-FDG PET/CT revealed a solitary hypermetabolic focus (maximum standardized uptake value of 9.8) restricted to the nasopharyngeal mass. biological workup and bone marrow analysis ruled out systemic plasma cell myeloma. A multidisciplinary tumor board recommended definitive external-beam radiotherapy using volumetric modulated arc therapy (VMAT) with daily onboard cone-beam computed tomography (CBCT) guidance to a total dose of 50 Gy in 25 fractions, intentionally omitting elective cervical nodal irradiation. The treatment was completed as planned without interruption or significant toxicity. Twelve weeks post-treatment, follow-up 18F-FDG PET/CT confirmed a complete metabolic response. Definitive fractionated radiotherapy using modern conformal techniques such as VMAT achieves excellent local control and complete metabolic response in localized nasopharyngeal SEP. Long-term structured surveillance remains imperative given the recognized risk of late systemic transformation to multiple myeloma.