NEURORADIOLOGY

Imaging Findings 

Study shows multiple hypointense lesions (T1WI), hyperintense on T2WI, with a few showing poor margins. All lesions are seen in close approximation to ventricular ependyma / subarachnoid spaces.

 Diagnosis:
Lymphoma 

Discussion   
Primary and Secondary Lymphoma

The incidence of CNS lymphoma is higher in the immunocompromised patient. There is no single uniform pattern of involvement by primary cerebral lymphoma, and patterns range from solitary or multiple, circumscribed nodules to diffuse infiltration or widespread distribution.

The hallmark of CNS lymphoma is its hyperdensity on NCCT, its periventricular localization, and its fluffy pattern of contrast enhancement (Fig 2). Almost all lymphomas are in direct contact with either the ventricular ependyma or the superficial subarachnoid space. Tumor margins are usually poorly defined, due to the characteristic perivascular infiltration pattern of the tumor cells. On CT, lesions are hyperdense with a homogenous enhancement, while on MR, they exhibit slight hypointensity on T1WI, and hypo to slightly hyperintense on T2WI and PD-WI, relative to gray matter. Common location is basal ganglia or temporal lobes, while for secondary lymphoma is the cerebral meninges.

In the AIDS patient, lymphomas are sometimes indistinguishable from other lesions, and the situation is further complicated by the frequent presence of combined pathology, i.e. toxoplasmosis and lymphoma.

Differentiation between lymphoma and Toxoplasmosis

Lymphoma is more likely when the enhancing lesion is subependymal, periventricular or in the corpus callosum. Rapid progression in short intervals also favors lymphoma. Enhancing lesions in the basal ganglia or the corticomedullary junction are more likely due to Toxoplasma gondii.

Standard practice for AIDS patients with enhancing CNS lesions by CT/MR and positive toxoplasma serology is an empirical cause of antitoxoplasmosis treatment for 10 to 21 days. If no clinical and/or radiographic response occurs, alternative diagnoses are considered more closely.

In AIDS patients with primary CNS lymphoma this approach has a significantly negative impact on morbidity and duration of survival, as these patients have only an average survival time of 4 months with radiation therapy and 1.5 months without radiation therapy. A fast and accurate differentiation between lymphoma and infectious causes is of vital importance for this patient group.

Role of functional MRI and SPECT:

Employing dynamic contrast fMRI, few researchers have found increased rCBV in regions with active lymphoma but reduced rCBV in the central regions of toxoplasmosis lesions. The rise within active lymphoma probably results from hypervascularity within the tumor, and the drop within toxoplasmosis lesions probably reflects lack of vasculature. This method of evaluation is useful in differentiating between the two.