Extranodal Marginal Zone Lymphoma of the Floor of the Mouth: A Rare Entity With Management Challenges

Extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) involving the oral cavity is rare, and localization to the floor of the mouth is particularly uncommon.Its indolent course and nonspecific clinical presentation may lead to diagnostic and therapeutic challenges.Diagnosis relies on histopathological examination combined with immunohistochemical analysis to establish the B-cell phenotype and exclude other small B-cell lymphomas.We report the case of a 48-year-old man with stage IIE MALT lymphoma of the floor of the mouth associated with ipsilateral submandibular lymph node involvement.Given the large tumor volume, systemic therapy was initially favored.The patient received four cycles of rituximab monotherapy with an insufficient response.A repeat biopsy excluded histological transformation, and rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisolone chemoimmunotherapy was subsequently administered, with persistent local disease.External radiotherapy was then delivered to the oral cavity and cervical lymph node regions (24 Gy), with a boost to the floor-ofmouth lesion to a total dose of 30 Gy. Follow-up positron emission tomography/computed tomography demonstrated a complete metabolic response, and the patient remained clinically free of recurrence 18 months after radiotherapy.This case highlights the diagnostic and therapeutic challenges of MALT lymphoma at an exceptional oral site and illustrates the importance of radiotherapy in achieving local disease control after an insufficient response to systemic therapy.