Patient presented with left sided hemiplegia.
There is a mass lesion that is isointense to hypointense to gray matter on the T2W and FLAIR images involving the parietal lobes, splenium and posterior body of the corpus callosum, hypothalamus and optic chiasm. Perilesional edema is noted. There is intense enhancement of this lesion after contrast administration.
Primary lymphoma of the CNS accounts for approximately 1% of all primary brain tumors. The site of origin of primary CNS lymphoma is unknown, since the CNS does not have endogenous lymphoid tissue nor a lymphatic circulation.
The people at risk are organ transplant recipients, patients with congenital immunodeficiency syndromes and those with AIDS and other systemic diseases associated with immunodeficiency. The peak age of incidence in the non-AIDS population is in the sixth decade. Nearly all primary CNS lymphoma is of the non-Hodgkin's type. When Hodgkin's lymphoma involves the brain parenchyma, it is almost always in the presence of systemic disease or with dural attachment. Focal masses are the most common initial presentation of primary CNS lymphoma, whereas the subarachnoid space is a common site for recurrent disease. Multiplicity is very common. Intracranial metastases from systemic lymphoma may be leptomeningeal (with or without parenchymal lesions) or dural-based.
Pathology: |
Pathologically, parenchymal lesions are ill-defined masses with irregular borders. Microscopically, the infiltrative edges of the lesions extend along perivascular spaces and infiltrate blood vessel walls. Dural-based masses often spread via the Virchow-Robin perivascular spaces and directly invade the brain parenchyma.
On MRI: |
Parenchymal masses may involve the deep gray matter structures, periventricular regions and corpus callosum. 75% of lymphomatous masses are seen to be in contact with ependyma, meninges, or both. The detection of enhancement along perivascular spaces on MRI should make one consider lymphoma, with sarcoidosis representing the only other consideration. The extent of edema on MRI is generally less than that seen in conjunction with primary gliomas or metastases of similar size. These masses do not calcify and hemorrhage is uncommon on imaging. MRI signal intensity patterns are extremely varied in focal intracerebral lymphoma, particularly in the AIDS population. These lesions, when deep and periventricular, are often isointense to gray matter on all pulse sequences. However, lymphoma can also be markedly hyperintense on T2W images. Enhancement after contrast administration occurs in the vast majority of cases and can be dense and homogeneous, but necrotic lesions can show ring enhancement. Lymphoma in AIDS patients more commonly appears as multifocal lesions with ring enhancement and with more prominent edema. These patients are often empirically treated with antitoxoplasmosis agents and followed for radiographic and clinical response in the absence of tissue diagnosis. Secondary brain involvement by systemic lymphoma is indistinguishable from primary CNS lymphoma on imaging studies.
Differential Diagnosis:
The differential diagnosis depends upon the appearance of the lesion. If there is a deep mass having relatively homogeneous intensity similar to gray matter, dense contrast enhancement and minimal edema, the most likely diagnosis is lymphoma. The single most valuable MR finding in the distinction between lymphoma and toxoplasmosis in AIDS is the presence of subependymal spread, which is highly suggestive of lymphoma. Hemorrhage is also very common in cerebral toxoplasmosis, particularly after treatment. If a focal enhancing parenchymal mass is accompanied by enhancement along the perivascular spaces, especially with adjacent meningeal enhancement, then lymphoma or sarcoidosis should be considered.
In the non-AIDS patient, other lesions are also in the differential diagnosis. Glioblastoma, like lymphoma, can cross the corpus callosum and enhance. Metastases are always in consideration, particularly when significant edema is noted. Parenchymal and extra-axial sarcoidosis would appear virtually identical to lymphoma. Densely enhancing dural-based lesions could implicate lymphoma, sarcoidosis, metastases (especially breast carcinoma) and meningioma (the only tumor to calcify amongst these).
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