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Primary CNS lymphoma

Dr Yuranga Weerakkody and Dr Frank Gaillard et al.

Primary CNS lymphoma (PCNSL) are uncommon tumours, accounting for only 1% of malignant CNS tumours.  By definition there is no co-existing systemic disease at the time of diagnosis, distinguishing it from CNS involvement from systemic lymphoma (secondary CNS lymphoma).

Epidemiology

Typically patients are over the age of 50 with short duration of symptoms (at most a few months) 4. There is a male predominance of approximately 2:1 4. As there is a strong association with HIV/AIDS and other immunocompromised states, demographics in these patients reflects the underlying condition. Approximately 2-6% of patients with HIV will develop PCNSL 5. 

Predisposing factors include:

Primary CNS lymphoma accounts for approximately 1% of all extranodal lymphoma, and ~1% of intracranial tumours. Recently there appears to be increaseing incidence at least partially due to increasing rates of immunocompromised patients.  An increase is however also seen in the non immunocompromised population.

Clinical presentation

Patients with PCNS lymphoma present similarly to patients with other central nervous system masses; symptoms and signs of raised intracranial pressure, focal neurological disorders and seizures. 

Pathology

Typically PCNSL present as multiple, infiltrating mass lesions that can arise in cortex, white matter or deep grey matter (more common in low grade lesions 1). They may demonstrate areas of necrosis especially in immunodeficient patients.

The vast majority (>90%) of PCNSL are B cell in origin: diffuse large B-cell lymphoma and high-grade Burkitt-like B-cell lymphoma 1. Malignant cells tend to accumulate around blood vessels.

Low-grade tumours are more frequently T-cell in origin 1. 

CSF examination demonstrates elevated protein and decreased glucose. Positive cytology is uncommon (~25%).

Radiographic features

Typically PCNSL are supratentorial (75-85%) 5 and appear as a mass or multiple masses (11-50% 3) that are usually in contact with the subarachnoid / ependymal surfaces. Crossing the corpus callosum is not infrequently seen. Enhancement on both CT and MRI is pronounced and usually homogenous. Even with larger lesions there is little mass effect for size, and limited surrounding vasogenic oedema. 

Low grade tumours differ from the more common high-grade PCNSL in several ways 1:

  • deep locations and spinal involvement is more common
  • contrast enhancement is absent, irregular or only mild

Disseminated meningeal / intraventricular disease is uncommon seen between 1 and 7% if cases at presentation and usually seen in high grade cases 8. 

CT
  • most lesions are hyper-attenuating (70%) 3
  • shows enhancement
  • haemorrhage is distinctly uncommon
  • there are often multiple lesions in patients with HIV / AIDS
MRI

Reported signal characteristics include

  • T1- typically hypo intense to white matter
  • T1 C+ (Gd) - typical high-grade tumours show strong homogenous enhancement while low grade tumours have absent to moderate enhancement 1
  • T2 - hyper intense 15-42% 1
    • more frequent in low-grade tumours
    • white matter hyperintensities (leukoencephalopathy): rare 5
  • MR spectroscopy
    • large choline peak
    • reversed choline/creatinine ratio
    • lactate peak may also be seen7
  • MR perfusion: shows only a modest increase in rCBV, much less marked than in high-grade gliomas, where angiogenesis is a prominent feature
Scintigraphy
Thallium 201
  • shows increased uptake
C11 Methionine PET
  • shows increased uptake

Treatment and prognosis

Treatment is predominantly with steroids (which can dramatically shrink the tumour) and chemotherapy (high dose methotrextate) with or without whole brain irradiation 4.

If the tumour is low grade (uncommon - see above) then local treatment with surgical resection and radiotherapy may be effective 1 and long term survival is possible.

The tumours are often high grade and despite treatment have a poor prognosis. If only surgical resection is performed then death occurs within a few months. With high dose chemotherapy the tumour can be significantly reduced in size, however recurrence is common, with median survival of around 30 months 1. Those who are immunocompromised (e.g HIV positive) do worse.

Differential diagnosis

For general imaging appearencs on CT and MRI consider

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