Primary CNS lymphoma (PCNSL)

PCNSL is the most common cause of mass lesions in children and the second most common in adults after toxoplasmosis. Histologically, this is a high-grade B cell lymphoma. The Ebstein-Barr virus can be isolated from tissue specimens and is believed to have a causal role in the development of the lymphoma.

The clinical presentation is similar to that of toxoplasmosis with focal neurological deficits such as hemiparesis and seizures. There are usually signs and symptoms of raised intracranial pressure with increasing headache, vomiting and papilloedema.

Although the isolation of EBV by PCR in the CSF is specific, most patients present with mass lesions and raised intracranial pressure. Lumbar puncture is therefore contraindicated. The CT and MRI findings may be

indistinguishable from those due to toxoplasmosis with multiple enhancing lesions with associated cerebral oedema and mass effect. However, a single lesion on MRI especially if the toxoplasma serology is negative, is more likely to be lymphoma, as are lesions which closely adhere to the ventricular walls.

The diagnosis of PCNSL is usually made by biopsy.This may be performed after failure of treatment with antitoxoplasma therapy for at least two weeks. However, since prognosis is poor even with whole brain radiotherapy, it is reasonable not to proceed with a biopsy unless there is a suspicion that other more treatable pathogies may be identified.