Clues:

1. Multicentric involvement of deep hemispheres
2. Association with immunosuppression
3. Rapid regression with corticosteroids / radiation therapy = “ghost tumor”

Prevalence: 0.3–2% of all intracranial tumors; 7–15% of all primary brain tumors (equivalent to meningioma + low-grade astrocytoma); M >F

Peak age: 30–50 years; M÷F = 2÷1

Histo: atypical pleomorphic B-cells mixed with reactive T-cells infiltrate blood vessel walls + cluster within perivascular (Virchow-Robin) spaces simulating vasculitis

• symptoms of rapidly enlarging mass (60%)
• symptoms of encephalitis (<25%), stroke (7%)
• cranial nerve palsy, demyelinating disease, motor dysfunction
• personality changes, headaches, seizures, cerebellar signs,
• CSF cytology positive in 4–25–43%: elevated protein, mononuclear / blast / other lymphoma cells

Location: supratentorial÷posterior fossa = 3–9÷1; paramedian structures preferentially affected; white matter + corpus callosum (55%), deep central gray matter of basal ganglia + thalamus + hypothalamus (17%), posterior fossa + cerebellum (11%), spinal cord (1%); multicentricity in 11–47%

Site: abuts ventricular ependyma + meninges (12–30%); “butterfly lymphoma” in frontal lobe involvement; dural involvement may mimic meningioma (rare)

Spread: typically infiltrating; may cross anatomic boundaries + midline (crosses corpus callosum); diffuse leptomeningeal spread; subependymal spread with ventricular encasement (= rim-lymphoma)

• Primary lymphoma is indistinguishable from secondary!
• commonly large discrete solitary lesion (57%)
  • A large lesion is suggestive of lymphoma!
• small + symmetric multiple nodular lesions (43–81%)
• diffusely infiltrating lesion with blurred margins
• spontaneous regression (UNIQUE feature)

CT:

• usually mildly hyperdense (33%) ← high nuclear-to-cytoplasmic ratio
• occasionally isodense / low-density area (least common)
• little mass effect with paucity of peritumoral edema

CECT:

• homogeneously dense + well-defined / irregular + patchy periventricular contrast enhancement
• commonly thick-walled ring enhancement in immuno-competent patient
• Steroids may inhibit contrast enhancement

MR (superior to CT):

• well-demarcated round / oval / gyral-shaped (rare) mass
• relatively little mass effect for size
• isointense / slightly hypointense (← high cell density) relative to gray matter on T1WI
• hypo- to isointense / hyperintense (less common) relative to gray matter on T2 / FLAIR (← high cellularity):
  • ring pattern (= central necrosis with densely cellular rim in hyperintense “sea of edema”) typical in immunocompromised patients
  • intense ring-shaped contrast enhancement on T1WI
• irregular sinuous / gyral-like contrast enhancement or homogeneous enhancement:
  • solid homogeneous enhancement in immunocompetent patient
  • irregular heterogeneous ringlike mass in immunocompromised patient
  • periventricular enhancement is highly SPECIFIC (DDx: CMV ependymitis)
• elevated choline levels on MR spectroscopy

Angio:

• avascular mass / tumor neovascularity
• focal blush in late arterial-to-capillary phase persisting well into venous phase
• arterial encasement
• dilated deep medullary veins

NUC:

• increased uptake of ²⁰¹Thallium on SPECT (100% sensitive, 93% specific)
• increased uptake of ¹¹C-methionine on PET

Prognosis: median survival of 45 days for AIDS patients; median survival of 3.3 months for immunocompetent patients; improved with radiation therapy (4.5–20 months) + chemotherapy

Rx: sensitive to radiation therapy

DDx:

1. Neoplastic disorders
   1. Glioma (may be bilateral with involvement of basal ganglia + corpus callosum, may show dense homogeneous enhancement with vascularity)
   2. Metastases (known primary, at gray-white matter junction)
   3. Primitive neuroectodermal tumor
   4. Meningioma
2. Infectious disease (multicentricity)
   1. Abscess, especially toxoplasmosis (large edema)
   2. Sarcoidosis
   3. Tuberculosis
3. Demyelinating disease
   1. Multiple sclerosis
   2. Progressive multifocal leukoencephalopathy