PCNSL Butterfly Lesion — SCE Infectious Diseases MCQ

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HardCNS InfectionPCNSL Butterfly LesionSCE Infectious Diseases

A 34-year-old man with HIV (CD4 540, VL <50 on DTG/TAF/FTC) develops new-onset seizures. MRI brain shows a solitary well-circumscribed enhancing mass in the corpus callosum crossing the midline ('butterfly' pattern). Toxoplasma IgG is negative. CSF cytology is pending. What is the most likely diagnosis?

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Correct answer: B — Primary CNS lymphoma — the butterfly pattern (bilateral corpus callosum involvement) with Toxoplasma IgG negativity strongly favours PCNSL over Toxoplasma

A solitary enhancing mass crossing the corpus callosum ('butterfly lesion') in an HIV patient with negative Toxoplasma IgG is highly suggestive of primary CNS lymphoma (PCNSL — usually EBV-driven DLBCL). Toxoplasma IgG negativity has >95% NPV for excluding Toxoplasma encephalitis. PCNSL typically appears as a single or few homogeneously enhancing periventricular lesions. CSF EBV PCR and cytology support diagnosis; stereotactic biopsy is definitive. Treatment is high-dose Methotrexate-based chemotherapy ± whole-brain radiotherapy. ART should be continued/initiated.

Reference: BHIVA 2022 – OI guidelines; NICE NG99 2018 – Brain tumours