Cerebral toxoplasmosis is an opportunistic central nervous system infection caused by the parasite Toxoplasma gondii. It is the most common intracranial mass lesion in immunosuppressed patients (especially HIV/AIDS — CD4 <100/mm³). Multiple, bilateral, ring-enhancing lesions are typical with predilection for the basal ganglia. The 'eccentric target sign' is a characteristic MRI finding. Negative thallium-201 SPECT is critically important for differentiation from primary CNS lymphoma. Dramatic response to anti-toxoplasma therapy is both diagnostic and therapeutic.
Age Range
20-60
Peak Age
35
Gender
Male predominant
Prevalence
Uncommon
Toxoplasma gondii is an obligate intracellular parasite transmitted through cat feces or contaminated meat. In immunocompetent individuals, after primary infection, the parasite remains dormant as bradyzoite forms in latent cysts in brain tissue (especially basal ganglia, corticomedullary junction, and white matter). When immunosuppression develops (CD4 <100/mm³), bradyzoites convert to active tachyzoite forms and produce necrotizing encephalitis. Coagulative necrosis develops in the center of lesions — this necrotic center appears as a non-enhancing hypointense zone on imaging. Around the necrotic area, an active inflammatory zone (vascular congestion, macrophage infiltration, edematous tissue) forms, showing ring enhancement due to blood-brain barrier disruption. The eccentric target sign forms when an asymmetric nodular enhancing area within the ring enhancement represents a zone of active parasite replication. Basal ganglia predilection is due to the high blood flow in this region and maximal accumulation of tachyzoites during hematogenous dissemination. Perilesional vasogenic edema appears as a hyperintense rim on T2/FLAIR from proteinaceous fluid leaking through the disrupted blood-brain barrier into the interstitial space.
An asymmetrically located nodular enhancing area within a ring-enhancing lesion — the most characteristic and diagnostically valuable MRI finding of toxoplasma encephalitis. This asymmetric nodule represents intense inflammation and vascular barrier disruption in the zone of most active parasite replication. It is a reliable finding for differentiation from homogeneous enhancement of primary CNS lymphoma and smooth ring enhancement of pyogenic abscess.
Post-gadolinium T1-weighted images demonstrate multiple ring-enhancing lesions. Ring enhancement is typically thin and irregular. The 'eccentric target sign' — an asymmetrically located nodular enhancing area within the ring enhancement — is characteristic of toxoplasmosis. Lesions are frequently located in the basal ganglia, thalamus, and corticomedullary junction. Lesion number is typically more than 2 with bilateral distribution.
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Multiple ring-enhancing lesions are observed in bilateral basal ganglia and corticomedullary junction, with an eccentric target sign present in one of the lesions.
On T2-weighted images, lesions show heterogeneous signal; necrotic center is hyperintense (fluid/necrotic debris), surrounding inflammatory rim is iso-hypointense (cellular tissue), and prominent perilesional vasogenic edema appears hyperintense. Edema may be disproportionate to lesion size and extends into surrounding white matter. In basal ganglia lesions, edema adjacent to the internal capsule and thalamus is notable.
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Heterogeneous signal lesions with prominent perilesional vasogenic edema are observed in the basal ganglia and corticomedullary junction on T2-weighted images.
Mild to moderate diffusion restriction may be seen at the lesion center on DWI, but differs from the marked central restriction in pyogenic abscess. ADC values are higher than pyogenic abscess (typically >1.0 × 10⁻³ mm²/s vs abscess <0.7). The inflammatory rim may show mild restriction. This difference is an important clue in differentiating toxoplasmosis from bacterial abscess.
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Mild to moderate diffusion restriction is observed at the lesion center on DWI, with ADC values higher than those expected for pyogenic abscess.
On FLAIR, lesions and perilesional edema demonstrate prominent hyperintense signal. CSF suppression provides better contrast for lesions adjacent to ventricles and edematous areas near cortical sulci. The number and distribution of multiple lesions are best evaluated on FLAIR. Subependymal spread may be detected as periventricular hyperintensity on FLAIR.
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Multiple hyperintense lesions with prominent perilesional edema are observed in the basal ganglia and corticomedullary junction on FLAIR.
On perfusion MRI (DSC-MRI), toxoplasma lesions demonstrate low rCBV (relative cerebral blood volume). rCBV in the lesion periphery and enhancing rim is low or equal to normal brain parenchyma. This finding is critically important for differentiation from primary CNS lymphoma (elevated rCBV) and glioblastoma (increased rCBV in surrounding tissue). Low rCBV reflects the infectious/inflammatory nature of the lesion without neovascularity.
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Low rCBV values are observed in the lesions and enhancing rim on perfusion MRI, with no findings suggestive of tumoral pathology containing neovascularity.
Multiple hypodense lesions are observed in the basal ganglia and corticomedullary junction on non-contrast CT. Additional hypodense areas in surrounding tissue are seen due to perilesional edema. Lesions are typically ill-defined with surrounding mass effect. Calcification may be seen in chronic cases (post-treatment). CT sensitivity is lower than MRI — small lesions or posterior fossa lesions may be missed.
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Multiple hypodense lesions with perilesional edema are observed in bilateral basal ganglia and corticomedullary junction on non-contrast CT.
MR Spectroscopy demonstrates prominent lipid (1.3 ppm) and lactate (1.33 ppm doublet) peaks in toxoplasma lesions — reflecting necrosis and anaerobic metabolism. Choline peak is low or normal (unlike proliferative tumor). NAA is markedly reduced (neuronal damage). The combination of lipid/lactate dominance with low choline supports infectious/necrotic process and aids differentiation from tumoral pathology (lymphoma) showing elevated choline.
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MR Spectroscopy demonstrates prominent lipid and lactate peaks, low choline, and reduced NAA in the lesions, findings consistent with necrotic/infectious process.
Criteria
Multiple (≥2) ring-enhancing basal ganglia lesions in HIV/AIDS patient with CD4 <100/mm³. Most common presentation. IgG antibodies are usually positive.
Distinct Features
Tendency for bilateral and symmetric distribution, eccentric target sign positive, prominent perilesional edema, low rCBV on perfusion. Dramatic response to anti-toxoplasma therapy in 2-3 weeks. Thallium-201 SPECT negative.
Criteria
Toxoplasmosis presenting as a single ring-enhancing lesion. Seen in approximately 15-20% of cases. Differential diagnosis with primary CNS lymphoma is particularly challenging.
Distinct Features
Thallium-201 SPECT, perfusion MRI, and MRS are critically important for differentiation from lymphoma. In toxoplasmosis: thallium negative, rCBV low, Cho low, lipid/lactate dominant. In lymphoma: thallium positive, rCBV high, Cho high. Empirical treatment response (2-3 weeks) is the most reliable differentiating criterion.
Criteria
Form accompanied by hemorrhagic component in toxoplasma lesions. Prominent hypointense blooming foci on SWI/T2* and methemoglobin-related hyperintense areas on T1.
Distinct Features
Hemorrhage in the necrotic center or inflammatory rim. Blooming artifact on SWI. T1 hyperintense signal (methemoglobin — not suppressed on fat suppression, differentiating from dermoid). Differential diagnosis with hemorrhagic metastasis or glioblastoma required. Clinical context (HIV/AIDS) and treatment response are distinguishing.
Distinguishing Feature
Primary CNS lymphoma typically shows homogeneous and intense enhancement (ring in toxoplasmosis), high rCBV on perfusion (low in toxoplasmosis), high choline on MRS (low in toxoplasmosis), marked restriction on DWI (mild in toxoplasmosis). Thallium-201 SPECT positive in lymphoma, negative in toxoplasmosis. More tendency to be a single lesion.
Distinguishing Feature
Pyogenic abscess shows marked central diffusion restriction on DWI (ADC <0.7 — purulent material), while toxoplasmosis shows mild restriction (ADC >1.0). Abscess ring enhancement is smoother and thinner. MRS in abscess shows amino acid peaks (valine, leucine, isoleucine — 0.9 ppm) and succinate (2.4 ppm) — different from lipid/lactate pattern in toxoplasmosis.
Distinguishing Feature
Metastases have known primary malignancy history, located at gray-white matter junction (not basal ganglia), increased rCBV on perfusion (low in toxoplasmosis). Metastases are typically seen in immunocompetent patients. Serology (Toxoplasma IgG) and clinical context (HIV/AIDS) aid differentiation.
Distinguishing Feature
Glioblastoma is typically a single large mass with irregular thick ring enhancement, markedly elevated rCBV on perfusion, high choline on MRS, infiltrative T2/FLAIR signal in surrounding parenchyma. Multiple smaller lesions, low rCBV, and response to anti-toxoplasma therapy distinguish toxoplasmosis. Glioblastoma is typically seen in immunocompetent patients.
Distinguishing Feature
Scolex within cyst (eccentric dot — 'dot sign') is pathognomonic in neurocysticercosis lesions, different from the eccentric target sign in toxoplasmosis. Neurocysticercosis is typically seen in immunocompetent patients in endemic regions. Calcified stage lesions are typical as small calcified dots on CT.
Urgency
urgentManagement
medicalBiopsy
Not NeededFollow-up
Empirik tedaviye 2-3 hafta sonra kontrol MR. Yanıt varsa 6 hafta tedavi devamı. Yanıt yoksa stereotaktik biyopsi (lenfoma ekartasyonu). Uzun süreli profilaksi CD4 >200 olana kadar.The treatment approach in cerebral toxoplasmosis is empiric antimicrobial therapy — biopsy is generally not needed. First-line treatment is pyrimethamine + sulfadiazine + folinic acid combination (6 weeks). Trimethoprim-sulfamethoxazole may be used as an alternative. Treatment response is evaluated as lesion shrinkage and edema reduction on MRI within 2-3 weeks — this dramatic response is also a diagnostic confirmation criterion. If no response to treatment, stereotactic brain biopsy is indicated (to exclude primary CNS lymphoma). Dexamethasone may be used to reduce perilesional edema and mass effect, but may complicate evaluation as it can also cause shrinkage in lymphoma. Secondary prophylaxis (low-dose trimethoprim-sulfamethoxazole) is maintained until CD4 >200/mm³. HAART (antiretroviral therapy) should be started concurrently.
CNS toxoplasmosis is the most common cause of focal brain lesions in HIV/AIDS patients. Clinical diagnosis is made by serology and imaging. Response to empiric therapy (pyrimethamine + sulfadiazine) is expected in 2-4 weeks; if no response, biopsy to exclude lymphoma is necessary. Immune reconstitution with antiretroviral therapy is essential.