A 37-year-old man with AIDS presents with confusion, lethargy and memory loss. CT of the brain demonstrates multiple supratentorial enhancing masses. Which imaging feature ours a diagnosis of toxoplasmosis rather than primary CNS lymphoma?
Correct Answer: Lesions without increased tracer activity on FDG-18 PET imaging
Description: Cerebral toxoplasmosis must be differentiated from other oppounistic infections or tumors in the CNS of AIDS patients. The differential diagnosis includes herpes simplex encephalitis, cryptococcal meningitis, progressive multifocal leukoencephalopathy, and primary CNS lymphoma. Toxoplasmosis - Noncontrast CT scans show multiple low-density areas. The differential diagnosis ors toxoplasmosis if there is associated hemorrhage, whereas the presence of subependymal extension with contrast enhancement of the ventricular walls ors lymphoma. In toxoplasmosis, Contrast CT scans may show no enhancement, nodular enhancement, or ringlike enhancement . MRI studies are more sensitive in detecting the Toxoplasma lesions than contrast-enhanced CT scans. They exhibit hypointensity on TI-weighted images and variable intensity on T2-weighted images. Gadolinium-enhanced MRI studies show enhancing patterns similar to those seen on contrast- enhanced CT scans.On PET imaging, increased tracer activity is seen in lymphoma as a result of hypermetabolic activity, but it is not seen in toxoplasmosis. On SPECT imaging, thallium 201 uptake also reflects hypermetabolism. On MRS, increased lipid and lactate peaks with a decrease in other metabolites are characteristic for toxoplasmosis. Lymphoma shows a large increase in choline peak with only a mild increase in lipid and lactate peaks. Primary CNS lymphoma - generally presents with focal neurologic deficits, including cranial nerve findings, headaches, and/or seizures. MRI or CT generally reveals a limited number (one to three) of 3 to 5 cm lesions. The lesions often show ring enhancement on contrast administration and may occur in any location. Contrast enhancement is usually less pronounced than that seen with toxoplasmosis. Differential diagnosis of ring-enhancing lesion include - metastasis, tuberculoma, neurocysticercosis, abscess, toxoplasmosis, CNS lymphoma, GBM.
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