Toxoplasmosis vs lymphoma

It is a relatively common occurrence for radiologists to be asked to distinguish between cerebral toxoplasmosis and primary CNS lymphoma (PCNSL) in patients with HIV/AIDS. Treatment is clearly different and thus accurate interpretation of CT and MRI is essential.

In many instances appearances are classic and pose little problem, however, in 50-80% of cases the appearances can be very similar requiring careful interpretation ¹. Below are helpful distinguishing features.

Radiographic features

Distribution

Primary CNS lymphoma typically demonstrates subependymal spread, whereas toxoplasmosis tends to be scattered through the basal ganglia and at the corticomedullary junction ¹. 

HIV lymphoma also is far more frequently a solitary lesion, whereas toxoplasmosis is usually multifocal (86%) ^2-3,3.

Enhancement

Following administration of contrast, on both CT and MRI, both entities enhance, however typically lymphoma is solid whereas toxoplasmosis demonstrates ring or nodular enhancement ^1-2,2. It should be noted however that in patients with HIV/AIDS, primary CNS lymphoma may also demonstrate ring enhancement. 

Haemorrhage

Haemorrhage does not happen typically in PCNSL before treatment but may be seen occasionally in toxoplasmosis, a finding that can help differentiate them.

MR spectroscopy

Although both entities have increased lactate and lipids, this tends to be less marked in lymphoma. Lymphoma typically demonstrates marked increase in Cho, whereas it is reduced in toxoplasmosis ^1-2,2. Both demonstrate decreased Cr and NAA. However, this pattern is variable.

MRS should be performed with both long and short TE sequences ¹.

MRI perfusion

A decrease in cerebral blood volume (rCBV) centrally within lesions suggests toxoplasmosis, whereas it is increased in lymphoma ¹. Unfortunately it is reduced in the perilesional oedema of both lesions.

SPECT

Thallium 201 ChlorideSPECT demonstrates increased uptake in lymphoma whereas it is decreased in toxoplasmosis ².

Practical points

Features that favour primary CNS lymphoma include:

• single lesion
• subependymal spread
• solid enhancement
• no haemorrhage before treatment
• Thallium SPECT positive
• MRS: increased choline (Cho)
• MR perfusion: increased rCBV

Features that favour cerebral toxoplasmosis include:

• multiple lesions
• scattered though basal ganglia and corticomedullary junction
• ring or nodular enhancement
• haemorrhage occasionally occurs mostly in periphery of lesion
• Thallium SPECT negative
• MRS: decreased choline (Cho)
• MR perfusion: decreased rCBV