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PCNSL Thallium SPECT — SCE Infectious Diseases MCQ

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HardCNS InfectionPCNSL Thallium SPECTSCE Infectious Diseases

A 45-year-old man with HIV develops a new brain lesion. MRI shows a single periventricular contrast-enhancing mass with restricted diffusion on DWI. Toxoplasma IgG is negative. CD4 is 480 cells/µL. CSF EBV PCR is positive at high viral load. Thallium SPECT shows increased uptake (Thallium avid). What is the most likely diagnosis?

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Correct answer: B — Primary CNS lymphoma (PCNSL) — single periventricular enhancing mass with positive CSF EBV PCR, Thallium avidity, negative Toxoplasma IgG, and high CD4 all favour PCNSL; stereotactic biopsy confirms

In this case, all diagnostic features converge on PCNSL: (1) periventricular location (classic for PCNSL), (2) negative Toxoplasma IgG (>95% NPV for Toxoplasma), (3) CD4 >200 (Toxoplasma almost never occurs above this), (4) positive CSF EBV PCR (EBV drives PCNSL in HIV), (5) Thallium SPECT avidity (PCNSL is metabolically active; Toxoplasma and PML are not Thallium-avid). Stereotactic biopsy is the definitive diagnostic step. Treatment: high-dose Methotrexate-based chemotherapy ± whole-brain radiotherapy. ART must continue.

Reference: BHIVA 2022 – HIV-associated malignancy; NICE NG99 2018