Differentiating Primary CNS Lymphoma from Toxoplasmosis

Cerebral toxoplasmosis and primary CNS lymphoma can present similarly both clinically and radiologically. PET and Thallium-201 brain SPECT can be used to help differentiate them.

Thallium-201 is sensitive for lesions larger than 2 cm. Initial and delayed (~4 hours) images are obtained. Lymphoma demonstrates increased activity on delayed images, while Toxoplasma demonstrates decreased activity over time.

With the advent of highly active antiretroviral therapy (HAART), however, new studies have found decreased specificity of Thallium-201 brain SPECT, presumably because of the improved immune response to Toxoplasmosis, resulting in increased uptake due to inflammatory cells. The increased numbers of patients with toxoplasmosis who demonstrate elevated uptake on Thallium-201 brain SPECT may make differentiation of toxoplasmosis and primary CNS lymphoma more difficult.

On PET, lymphoma shows high standardized uptake values (usually more than 3.5), while toxoplasmosis usually does not. PET has higher resolution than SPECT and can be used for smaller lesions. Of note: corticosteroid treatment may decrease F-18 FDG activity on PET.

References

• Giancola ML, Rizzi EB, Schiavo R, Lorenzini P, Schininà V, Alba L, Del Grosso B, Gigli B, Rosati S, Mango L, Bibbolino C, Antinori A. Reduced value of thallium-201 single-photon emission computed tomography in the management of HIV-related focal brain lesions in the era of highly active antiretroviral therapy. AIDS Res Hum Retroviruses. 2004 Jun;20(6):584-8.
• Lorberboym M, et al. Rapid Differential Diagnosis of Cerebral Toxoplasmosis and Primary Central Nervous System Lymphoma by Thallium-201 SPECT. The Journal of Nuclear Medicine. 1996 37 (7);1150-1154.