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45 year old male with HIV and aphasia.

Abscess

Metastasis

Infarcts

MS/demyelinating plaques

Primary neoplasm

Radiation necrosis

Resolving hematomas.

Abscess

Metastasis

Infarcts

MS/demyelinating plaques

Primary neoplasm

Radiation necrosis

Resolving hematomas.

Toxoplasma gondii

Ubiquitous protozoan

Obligate intracellular parasite

20-70% of US population is seropositive

Most common CNS infection of AIDS (other than HIV itself)

Infects 3-40% of AIDS population

CD4 T-cell counts <200

Acute disease in immunocompromised hosts typically reactivation of remotely acquired latent infection.

Transmission

primarily by ingestion of undercooked pork that contains tissue cysts

by exposure to oocysts either through ingestion of contaminated vegetables or direct contact with cat feces

transplacental route, blood product transfusion, and organ transplantation

Presentation

subacute onset with focal neurologic abnormalities frequently accompanied by headache, altered mental status, and fever

motor weakness, speech disturbances, seizures.

Imaging

Most common cause of a focal lesion in HIV+

Multiple, variable size

Basal ganglia (75%), thalamus, G-W junction

Ring-enhancement

May be hemorrhagic

May calcify after treatment.

Lymphoma is the second most common cause of focal lesion in HIV+

Improvement on therapy constitutes empiric evidence of toxoplasmosis

70-95% response on antibiotic regimen

Biopsy classically considered if no response in 10-14 days

Difference in frequency between these two entities has diminished with the use of Toxoplasmosis prophylaxis

concomitant use of steroids during therapy trial may falsely suggest that a lymphoma has responded to Toxoplasma therapy

Potential morbidity associated with delayed diagnosis of lymphoma

Clinical importance of differentiating Toxo from lymphoma

CT/MR Imaging is indeterminant.

Both may produce solitary or multiple lesions

Toxo tends to have more numerous and smaller lesions

Location

Toxo -- basal ganglia

Lymphoma -- periventricular/subependymal

CT

Lymphoma -- high-density lesions

Nuclear Medicine

Thallium201-SPECT -- uptake with lymphoma

PET -- lymphoma is hypermetabolic

MR diffusion

Diffusion is restricted in lymphoma>>toxo

contradistinction to the restricted diffusion within typical brain abscesses

MR spectroscopy

Lymphoma -- elevated choline peak.