CNS Atlas
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     Key features:
Clinical: Primary CNS lymphomas typically occur in otherwise healthy individuals over 40 yrs of age, or in younger immunosuppressed patients
Imaging: typically shows homogeneously enhancing periventricular mass lesion, 40%  multifocal.  Necrosis is more common in immunosuppressed patients.
Location: primary CNS lymphomas typically involve the cerebral parenchyma, most commonly the periventricular white matter or basal ganglia.  Systemic lymphomas secondarily involving the brain typically go to the subarachnoid space.
Histopathology: large majority are diffuse large B-cell lymphomas.  Discohesive, large, cytologically atypical, lymphoid cells form solid sheets or infiltrate brain parenchyma, perivascular spaces and blood vessel walls.

Malignant lymphoma arising in the brain and typically not, at least initially, associated with systemic lymphoma (although 5 to 10% may metastasize). Characterized by large, atypical lymphoid cells infiltrating cerebral parenchyma and blood vessel walls and almost always expressing a B-cell phenotype. Often associated with an immunodeficient state such as HIV.

Differential diagnosis:
Metastatic tumors

Primary CNS lymphomas may arise from lymphocytes that specifically home to the CNS; however, markers that distinquish CNS lymphomas from systemic lymphomas have not been identified

Primary CNS lymphomas typically occur in otherwise healthy individuals over 40 yrs of age, or in younger immunosuppressed patients
Typically present with focal neurologic signs and symptoms, neuropsychiatric disturbances or increased intracranial pressure. Seizures are less common.
5 to 10% of cases metastesize and involve extracerebral sites
Rare intravascular lymphomas present with rapidly progressive dementia and multiple neurologic signs ansd symptoms

Primary lymphomas in older non-immunesuppressed patients are typically homogeneously enhancing, often periventricular or within deep gray nuclei, and multiple in 40% of cases.
Primary lymphomas in younger immune suppressed patients may be ring enhancing, correlating with the presence of central necrosis histologically.
Treatment with steroids may cause lymphomas to disappear from CT scans
Sixty to 90% of primary CNS lymphomas are supratentorial, often crossing the corpus callosum.

Large majority of cases are diffuse large B-cell lymphomas
Single cell or small group infiltration of cerebral parenchyma, and infiltration into perivascular spaces and blood vessel walls, is typical.
Glioma cells may be perivascular but do not infiltrate into blood vessel walls. Metastatic carcinomas do not usually show single cell infitration.
Cytologic features are typical, including a coarse pepper-like chromatin pattern, nuclear grooves and irregular nuclear outlines.
Reactive lymphocytes and foamy macrophages may be present. Occasional biopsy specimens taken from the periphery of the lesion are non-diagnostic, showing only these reactive elements.

Rare intravascular lymphomas are limited to intravascular lumens. In these cases, peripheral blood is not involved (i.e., this is not a leukemia). Systemin organs such as skin and lung may also be involved.

Smear or touch preparations performed at the time of frozen section show typical cytologic features of malignant lymphoma with single cells, discrete cell margins with an absence of glial processes.
A coarse pepper-like chromatin pattern, nuclear grooves and irregular nuclear outlines are typical.
CSF cytology is positive in a low percentage of cases of primary CNS lymphoma, 0 to 30% in different series.
CSF cytology is positive in a large majority of systemic lymphomas that secondarily involve the brain and subarachnoid space.

95% 0f cases are B-cell lymphomas. Larger malignant cells stain positive for CD20 (L26), while small non-neoplastic T-cells stain positive for CD3.
More than 95% of cases associated with immunesupression are associated with evidence of EBV infection. This is rare in non-immune supressed patients.
Mib-1 proliferative rate is high, often approx. 50%

Systemic diffuse large B-cell lymphomas are asssociated with long term survival or cure in approx, 50% of cases. In contrast, primary CNS lymphomas have, until recently, have been associated with an exceptionally poor prognosis (average survival of approx. 1 year, similar to that of glioblastoma). This discrepancy may be related to the inability of chemotherapeutic agents to cross the blood brain barier
Recently developed chemotherapeutic protocols have been associated with significantly prolonged survival times for primary CNS lymphpomas (average survival of up to 4 years)
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Lymphoma: cytologically malignant lymphocytes infiltrating cerebral parenchyma and blood vessel wall
Lymphoma: typical cytologic features of large B-cell lymphoma
Lymphoma: infiltration of cerebral parenchyma and blood vessel walls
Lymphoma: Neoplastic lymphocytes infiltrating cerebral parenchyma and blood vessel wall
lymphoma: Neoplastic lymphocytes infiltrating blood vessel wall and cerebral parenchyma
Primary CNS lymphoma: parenchymal and perivascular infiltration
Lymphoma: Cytologically malignant lymphocytess infiltrating blood vessel wall
Lymphoma: infiltration within blood vessel wall
Lymphoma: infiltration within vessel wall and perivascular space
Primary CNS lymphoma: T-cell lymphomas such as this one account for less than 5% of primary CNS lymphomas
Lymphoma: fresh tissue preparation showing single cells, absence of glial processes
Lymphoma: CD20 immunohistochemistry shows positive immunostaining in larger neoplastic B lymphocytes
Primary CNS lymphoma: CD19 B-cell inmmunostain
Intravascular lymphoma
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