What are the key imaging characteristics of primary CNS lymphoma in patients with AIDS, and how can it be differentiated from cerebral toxoplasmosis on MRI?

Primary CNS lymphoma is the most common intracranial tumor in AIDS patients, presenting as solitary or multiple enhancing mass lesions, often centrally located within deep white matter or basal ganglia. These lesions typically appear T2 isointense to white matter with homogeneous enhancement. In contrast, toxoplasmosis lesions are usually T2 hypointense and may show ring enhancement. Key diagnostic features include restricted diffusion on DWI and increased perfusion on perfusion MRI for lymphoma. Clinical pearls: Consider lymphoma in patients with negative toxoplasmosis serology or poor response to empiric anti-toxoplasmosis therapy. Common pitfalls include misdiagnosing lymphoma as toxoplasmosis due to overlapping imaging features.