Atypical orbital mucosa-associated lymphoid tissue lymphoma involving the inferior rectus in a young adult: A case report and literature review.

[RATIONALE] To report a rare case of orbital mucosa-associated lymphoid tissue (MALT) lymphoma presenting in a relatively young adult with inferior rectus involvement, and to review the literature on this atypical presentation.

[PATIENT CONCERNS] A 46-year-old man presented with progressive swelling of the right lower eyelid, proptosis, and restricted eye movement.

[DIAGNOSES] Orbital magnetic resonance imaging revealed a well-defined, homogeneously enhancing mass involving the inferior rectus. Histopathology confirmed MALT lymphoma with immunophenotyping positive for CD20, BCL2, and myeloid cell nuclear differentiation antigen, and negative for CD5, CD10, and Cyclin D1.

[INTERVENTIONS] The patient underwent orbital biopsy via the lamina papyracea, allowing safe access to the inferomedial orbit. He then received definitive radiotherapy (24 Gy in 16 fractions) following National Comprehensive Cancer Network guidelines.

[OUTCOMES] The patient showed a good early metabolic response to orbital radiotherapy on post-treatment positron emission tomography, without significant acute adverse effects, and was scheduled for ongoing surveillance.

[LESSONS] This case illustrates an unusual demographic and anatomical presentation of orbital MALT lymphoma. The lamina papyracea route enabled high-quality tissue acquisition in a delicate periocular region, aiding accurate diagnosis. Literature review highlights the predominance of older female patients and superolateral orbital involvement, contrasting with this younger male case with inferior rectus infiltration.