Toxoplasmosis vs lymphoma

It is common for radiologists to be asked to distinguish between cerebral toxoplasmosisToxoplasmosis and primary CNS lymphoma (PCNSL) inlymphoma are frequently differential diagnoses in patients with HIV/AIDS. Treatment and as treatment is clearlysubstantially different and thus accurate interpretation of CT and MRIdistinguishing the two is essentialimportant.

In many instances, the imaging appearance is classic and differentiation is not problematic; however, in 50-80% of cases, the appearances can be very similar ¹. Below are helpful distinguishing features.

For a general discussion on each diagnosis, please refer to the individual articles: cerebral toxoplasmosis and primary CNS lymphoma.

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.

Enhancement

On CT and MRI, both entities enhance following administration of contrast. Lymphoma may solidly enhance, whereas toxoplasmosis usually demonstrates ring or nodular enhancement ^1,2.

However, in the setting of HIV/AIDS, primary CNS lymphoma may also demonstrate ringperipheral enhancement. Thus, the pattern of enhancement may not be helpful.

Haemorrhage

Haemorrhage does not happen typicallyis uncommon in PCNSL beforelymphoma, especially prior to treatment, but may be seen occasionally in toxoplasmosis.

MR spectroscopy

• both entities demonstrate increased lactate and lipids, although this tends to be less marked in lymphoma
• lymphoma typically demonstrates marked increase in Chocholine, whereas it is reduced in toxoplasmosis ^1,2
• both lesions demonstrate decreased Crcreatine and NAA; however, this finding is variable

Ideally, MRSMR spectroscopy (MRS) should 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. However, rCBV is reduced in the perilesional oedema of both lesions.

SPECT

Thallium 201 ChloridechlorideSPECT SPECT demonstrates increased uptake in lymphoma, because thallium serves as a potassium analogue and is avidly taken up by hypermetabolic tumortumour cells ⁶.  By contrast, thallium activity is decreased in toxoplasmosis because there is no cellular correlate 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 thoughthroughout the basal ganglia and corticomedullary junction
• ring or nodular enhancement
• haemorrhage occasionally occurs mostly in the periphery of the lesion
• Thallium SPECT negative
• MRS: decreased choline (Cho)
• MR perfusion: decreased rCBV