Toxoplasmosis vs lymphoma
It is common 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, 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.
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.
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 ring enhancement. Thus, the pattern of enhancement may not be helpful.
Hemorrhage does not happen typically in PCNSL before treatment, but may be seen occasionally in toxoplasmosis.
- both entities demonstrate increased lactate and lipids, although this tends to be less marked in lymphoma
- lymphoma typically demonstrates marked increase in Cho, whereas it is reduced in toxoplasmosis 1,2
- both lesions demonstrate decreased Cr and NAA; however, this finding is variable
MRS should be performed with both long and short TE sequences 1.
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 edema of both lesions.
Thallium 201 Chloride SPECT demonstrates increased uptake in lymphoma, because thallium serves as a potassium analog and is avidly taken up by hypermetabolic tumor cells 6. By contrast, thallium activity is decreased in toxoplasmosis because there is no cellular correlate 2.
Features that favor primary CNS lymphoma include:
- single lesion
- subependymal spread
- solid enhancement
- no hemorrhage before treatment
- Thallium SPECT positive
- MRS: increased choline (Cho)
- MR perfusion: increased rCBV
Features that favor cerebral toxoplasmosis include:
- multiple lesions
- scattered though basal ganglia and corticomedullary junction
- ring or nodular enhancement
- hemorrhage occasionally occurs mostly in periphery of lesion
- Thallium SPECT negative
- MRS: decreased choline (Cho)
- MR perfusion: decreased rCBV