Pictorial ReviewLymphomatous involvement of the central nervous system
Introduction
Lymphoma may arise within the central nervous system (CNS), known as primary CNS lymphoma (PCNSL) or may involve the CNS secondary to systemic disease. PCNSL, by definition is limited to the brain, leptomeninges, spinal cord, or eyes without any evidence of spread outside the CNS at time of diagnosis.1 It accounts for up to 5% of all brain tumours and approximately 1% of all non-Hodgkin lymphomas (NHLs).2, 3 Patients with both congenital (e.g., Wiskott–Aldrich syndrome, X-linked lymphoproliferative syndrome, IgA deficiency) and acquired immunosuppression (human immunodeficiency virus [HIV]/acquired immunodeficiency syndrome [AIDS], post-bone marrow and solid-organ transplantation) show increased incidence of PCNSL.4 Of these, AIDS is the most important risk factor, with up to 6% of these patients developing PCNSL, although the incidence has reduced post-highly active anti-retroviral therapy (HAART).1, 5 Nevertheless, PCNSL remains an AIDS-defining diagnosis.6 In post-transplant patients, the incidence is highest during the first year, in patients with heart and lung transplants and in children younger than 10 years.7 Histopathologically, >90% of PCNSLs are histologically diffuse large B-cell lymphomas with low-grade Burkitt's or T-cell lymphomas accounting for the rest.6, 8
Most immunocompetent patients present in the fifth to seventh decades of life, with immunocompromised patients presenting much earlier, in the third to fourth decades.9, 10, 11 There is slight male predilection in immunocompetent patients (male:female ratio 1.2:1) and significant male predilection in immunocompromised patients (9:1)9, 12, 13. The later likely reflects a historical bias from data of HIV-positive patients, and a similar predilection is not seen post-transplant.7, 9 Patients may present with altered mental status, headache or visual, sensory, or motor deficits.7, 12, 13 A provisional diagnosis of PCNSL can be made on imaging, potentially changing the management algorithm from neurosurgical resection to biopsy, as surgical resection does not improve outcome and chemotherapy remains the mainstay for treatment.13, 14, 15
Section snippets
PCNSL in immunocompetent patients
The cerebral hemispheres (20–43%), in particular the frontal lobes, are the commonest sites of involvement, with the deep grey matter nuclei (13–20%), corpus callosum (14%), and periventricular regions (12%) involved less frequently.14 The occurrence of solitary and multiple lesions has been variably reported between 40–80% and 20–40% in the literature, respectively.1, 14, 16, 17 Over 95% of cases demonstrate at least one intra-axial lesion contacting a cerebrospinal fluid (CSF) surface (either
PCNSL in immunocompromised patients
In this group of patients, the majority of tumours are supratentorial (75%), with the basal ganglia (40–53%) being most frequently involved followed by the frontal lobe.1, 5, 9, 10 Lesions are often multifocal (30–80%), necrotic, and frequently exhibit heterogeneous enhancement.1
At CT, lesions are often iso- to hypodense (up to 80%), in contrast to immunocompetent patients where lesions are frequently hyperdense.1, 5 Tumours are predominantly iso- to hypointense on T1WI and iso- to hyperintense
Secondary CNSL
Secondary CNS involvement occurs in up to 10–15% cases of systemic lymphoma and tends to occur early, often between 5–12 months after the primary diagnosis.1, 23 Alternately, CNS involvement may herald disease relapse. Secondary CNSL most commonly involves the leptomeninges, often without any referable symptoms.23 The meningeal enhancement, however, may not be discernible on imaging and progressive hydrocephalus may be the only sign.
In evaluating leptomeningeal disease, CT is suboptimal, with
Primary meningeal lymphoma
PCNSL may rarely arise from the dura or the leptomeninges. The primary dural subtype has an estimated incidence of 2.4%, predominantly affecting middle-aged women in the fifth decade.24 Histologically, it is often a low-grade marginal zone type lymphoma in contrast to PCNSL24
Lesions are solitary in 50% of cases. On imaging, they mimic meningiomas and may be hyperdense on CT and show bony hyperostosis. At MRI, they often enhance diffusely and may show dural tail8 (Fig 10). Coexistent
Conclusion
PCNSLs can have varying imaging appearances, predominantly dictated by the immune status of the patient. Barring some overlap, PCNSL in immunocompetent patients generally presents late, is solid, DWI bright, and shows uniform enhancement. Contiguity with a CSF surface and perivascular enhancement are useful clues. Immunocompromised patients, on the other hand, present earlier and often have multiple, necrotic, haemorrhagic lesions with irregular or rim enhancement. Secondary CNS involvement
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