Primary CNS Lymphoma

Primary central nervous system lymphoma (PCNSL) is a rare and aggressive form of non-Hodgkin lymphoma confined to the brain, spinal cord, leptomeninges, and/or eyes without evidence of systemic disease. The vast majority (>90%) are diffuse large B-cell lymphoma (DLBCL). PCNSL presents unique diagnostic and therapeutic challenges: it requires a distinct diagnostic approach (stereotactic biopsy rather than resection), responds dramatically but transiently to corticosteroids (the “ghost tumor” phenomenon), and demands chemotherapy regimens built around high-dose methotrexate that can penetrate the blood-brain barrier. The treatment landscape has evolved significantly, with the adoption of autologous stem cell transplant (ASCT) as consolidation and a declining role for whole-brain radiation therapy (WBRT) due to its devastating delayed neurotoxicity. Modern protocols achieve 5-year survival rates exceeding 50%, a marked improvement over historical outcomes.

Bottom Line

Histology: >90% diffuse large B-cell lymphoma (DLBCL); arises de novo in the CNS without systemic lymphoma
Epidemiology: Two populations — immunocompetent elderly (median age 65, rising incidence) and immunocompromised (HIV, transplant recipients, EBV-driven)
Imaging: Periventricular/deep white matter masses; homogeneously enhancing in immunocompetent, ring-enhancing in immunocompromised; restricted diffusion; may cross midline
“Ghost tumor”: Corticosteroids cause rapid tumor regression (lympholytic effect) — AVOID steroids before biopsy whenever possible, as they can render biopsy non-diagnostic
Diagnosis: Stereotactic biopsy (NOT resection); surgery does not improve outcomes; CSF cytology + flow cytometry; mandatory slit lamp exam for ocular involvement
Treatment backbone: High-dose methotrexate (HD-MTX) ≥3.5 g/m² — the single most important agent in PCNSL
Consolidation: Autologous stem cell transplant (ASCT) with thiotepa-based conditioning is the preferred approach (IELSG32); WBRT declining due to severe delayed leukoencephalopathy
Prognosis: Median OS >5 years with modern regimens; IELSG prognostic score guides risk stratification

Epidemiology

Immunocompetent Population

Median age at diagnosis: ~65 years; slight male predominance
Incidence: ~0.4–0.5 per 100,000 person-years; has been increasing in the elderly over the past two decades
Accounts for 2–3% of all primary CNS tumors and 4–6% of all extranodal lymphomas
No clear environmental risk factors identified; not associated with EBV in the immunocompetent

Immunocompromised Population

Risk Group	Mechanism	EBV Association	Key Features
HIV/AIDS	Severe CD4+ T-cell depletion (<50 cells/μL)	>95% EBV-positive	Incidence has declined dramatically with antiretroviral therapy (ART); ring-enhancing lesions (vs toxoplasmosis)
Organ transplant	Iatrogenic immunosuppression	Often EBV-positive	Part of post-transplant lymphoproliferative disorder (PTLD) spectrum; may respond to immunosuppression reduction
Autoimmune disease	Chronic immunosuppressive therapy	Variable	Increasing recognition; may be related to specific immunosuppressants (e.g., methotrexate, TNF inhibitors)
Primary immunodeficiency	Congenital immune defects	Often EBV-positive	Rare; typically presents in childhood or young adulthood

Clinical Presentation

Neurologic Symptoms

Focal neurologic deficits: Most common presentation (~70%); hemiparesis, aphasia, or other deficits depending on tumor location
Neuropsychiatric symptoms: Personality changes, cognitive decline, psychomotor slowing (~40%); reflects the predilection for deep white matter and periventricular regions
Increased intracranial pressure: Headache, nausea, papilledema (~30%)
Seizures: Less common than with other brain tumors (~15%), likely because PCNSL tends to involve deep structures rather than cortex
Ocular symptoms: Blurred vision, floaters, decreased visual acuity (~15–20% at diagnosis; up to 25% develop during disease course)

Sites of CNS Involvement

Location	Frequency	Clinical Significance
Cerebral hemispheres (periventricular, deep white matter, basal ganglia)	~60%	Most common site; predilection for periventricular regions is characteristic
Corpus callosum	~15%	May cross midline (“butterfly pattern”); similar to GBM but enhances homogeneously
Posterior fossa	~15%	Cerebellum or brainstem involvement
Eyes (vitreous, retina, optic nerve)	~15–25%	Primary vitreoretinal lymphoma may precede or accompany CNS disease; bilateral in 80%
Leptomeninges/CSF	~15–20%	Positive CSF cytology or flow cytometry; cranial nerve palsies, radiculopathy
Spinal cord (intramedullary)	<5%	Rare; may present as progressive myelopathy

Imaging

MRI Characteristics

Characteristic MRI Findings by Immune Status

Immunocompetent patients:
- Homogeneously enhancing lesion(s); solitary in ~60%, multifocal in ~40%
- Periventricular, deep white matter, basal ganglia, corpus callosum predilection
- Restricted diffusion (high cellularity) — distinguishes from many other brain tumors
- Iso- to hypointense on T2/FLAIR (unlike most gliomas which are T2-hyperintense)
- May cross the midline through the corpus callosum
- Relatively little surrounding edema for the size of the lesion (compared with metastases)
Immunocompromised patients:
- Ring-enhancing pattern more common (central necrosis)
- May be multifocal
- Major differential: cerebral toxoplasmosis (in HIV/AIDS)
- Thallium SPECT or FDG-PET can help distinguish lymphoma (high uptake) from toxoplasmosis (low uptake)

Imaging Differential Diagnosis

Diagnosis	Distinguishing Features
PCNSL	Periventricular, homogeneous enhancement, restricted diffusion, iso/hypointense T2, contacts ependymal surface
High-grade glioma (GBM)	Heterogeneous ring enhancement, necrotic core, T2-hyperintense infiltrative component, rarely restricted diffusion
Metastasis	Gray-white junction, known primary, disproportionate edema, often multiple
Toxoplasmosis (HIV)	Ring-enhancing, basal ganglia, multiple, eccentric target sign; low thallium SPECT uptake
Tumefactive demyelination	Incomplete ring enhancement (“open ring”), young patient, may have leading edge of restricted diffusion
Sarcoidosis	Leptomeningeal and dural enhancement, cranial nerve involvement, systemic features

The “Ghost Tumor” Phenomenon

Corticosteroid Effect on PCNSL

Corticosteroids have a direct lympholytic and pro-apoptotic effect on lymphoma cells, causing rapid tumor regression — often within 24–48 hours
The tumor may completely disappear on imaging (“ghost tumor” or “vanishing tumor”), rendering subsequent biopsy non-diagnostic
Critical rule: If PCNSL is in the differential diagnosis, withhold corticosteroids until after biopsy whenever safely possible
Exception: corticosteroids should not be withheld if there is imminent herniation, severe mass effect, or other life-threatening situation
If steroids have already been given: delay biopsy until the tumor re-grows after steroid taper (may take 2–6 weeks); alternatively, proceed with biopsy understanding that diagnostic yield may be reduced
Steroid-induced tumor regression does NOT indicate cure — PCNSL invariably recurs without definitive treatment

Diagnostic Workup

Tissue Diagnosis

Stereotactic biopsy: The standard approach; diagnostic yield >90% when steroids have not been administered
Surgical resection is NOT recommended: Multiple randomized and retrospective studies have shown that extent of resection does NOT improve survival in PCNSL, unlike in most other brain tumors; resection adds surgical morbidity without benefit
Histopathology: Angiocentric pattern of perivascular lymphoid infiltrate; immunohistochemistry shows CD20+, CD79a+ B cells; MYD88 L265P mutation present in ~70% of PCNSL (characteristic but not specific)

Complete Staging Workup

Required Workup for Suspected PCNSL

MRI brain and spine (with contrast): Assess full extent of CNS disease; spinal involvement in ~5%
CT chest, abdomen, pelvis: Exclude systemic lymphoma (mandatory — by definition, PCNSL has no systemic disease)
PET-CT (body): More sensitive than CT for detecting occult systemic lymphoma; increasingly used
Lumbar puncture: CSF cytology, flow cytometry (CD19, CD20, kappa/lambda light chains), protein, glucose, cell count; positive in ~15–20% at diagnosis
Slit lamp examination (ophthalmologic): Mandatory in all patients; vitreous involvement present in 15–25%; bilateral in ~80% when present
Testicular ultrasound (in men): Testicular DLBCL has a unique tropism for the CNS; must be excluded as a primary site
HIV testing: All patients; determines treatment approach and prognosis
Bone marrow biopsy: To exclude systemic lymphoma involvement (included in many protocols)
Complete blood count, LDH, comprehensive metabolic panel, β2-microglobulin: Baseline and prognostic

CSF Analysis

Cytology: Malignant lymphocytes in ~15–20% at diagnosis; repeat LP increases sensitivity
Flow cytometry: More sensitive than cytology; detects monoclonal B-cell populations (light chain restriction)
MYD88 L265P mutation (cell-free DNA): Emerging biomarker; can be detected in CSF even when cytology is negative; sensitivity ~50–70%
Protein: Elevated in most cases
IL-10 and IL-10/IL-6 ratio: Elevated IL-10 and elevated IL-10/IL-6 ratio support the diagnosis; useful when cytology is negative

Vitreoretinal Involvement

Primary vitreoretinal lymphoma (PVRL) is considered a variant of PCNSL; ~80% of PVRL patients eventually develop brain involvement
Presentation: Floaters, blurred vision, decreased acuity; often misdiagnosed as uveitis for months before diagnosis
Slit lamp findings: Vitreous cellular infiltrate, subretinal deposits (“leopard spots”)
Diagnosis: Vitreous biopsy with cytology, flow cytometry, IL-10/IL-6 ratio, MYD88 mutation analysis
Treatment: Systemic HD-MTX (treats both CNS and ocular disease); intravitreal methotrexate or rituximab for isolated or refractory ocular disease

Treatment

Induction Chemotherapy

The cornerstone of PCNSL treatment is high-dose methotrexate (HD-MTX), which is the only agent with consistent level 1 evidence in this disease. Adequate CNS penetration requires doses ≥3.5 g/m² (typically 3.5–8 g/m²) delivered as a rapid infusion over 2–4 hours:

Regimen	Components	Key Trial	ORR / CR Rate
HD-MTX monotherapy	Methotrexate 3.5–8 g/m² every 2 weeks	Multiple single-arm studies	ORR ~50–70%; CR ~30–40%
MTX + rituximab	HD-MTX + rituximab	HOVON 105	Addition of rituximab did not significantly improve outcomes in randomized trial
MATRix	MTX + cytarabine + thiotepa + rituximab	IELSG32 (phase 2)	CR ~49%; best response with 4-drug combination
R-MBVP	Rituximab + MTX + BCNU + VP-16 + prednisone	LOC network	CR ~46%
R-MPV	Rituximab + MTX + procarbazine + vincristine	MSKCC phase 2	ORR ~95%; CR ~60% (followed by reduced-dose WBRT)

Methotrexate Administration and Toxicity

Dose: ≥3.5 g/m² IV over 2–4 hours (rapid infusion maximizes CSF levels)
Leucovorin rescue: Mandatory; begun 24 hours after MTX infusion and continued until serum MTX level <0.05 μmol/L
Hydration and urinary alkalinization: Essential to prevent MTX crystallization in renal tubules; urine pH must be maintained ≥7.0
Renal toxicity: Most serious acute complication; monitor creatinine before each cycle; hold if creatinine clearance <50 mL/min
Drug interactions: Avoid NSAIDs, penicillins, proton pump inhibitors, and other drugs that reduce MTX clearance; hold for at least 48 hours before and after MTX
Mucositis: Common; dose-limiting in some patients
Age consideration: HD-MTX is tolerable in patients up to 75–80 years with adequate renal function; age alone is not a contraindication

Consolidation Therapy

Consolidation Strategy	Approach	Key Evidence	Advantages / Disadvantages
ASCT with thiotepa-based conditioning	High-dose chemotherapy (thiotepa + BCNU or busulfan) followed by autologous stem cell rescue	IELSG32: PFS superior to WBRT; comparable OS	Best long-term tumor control without delayed neurotoxicity; limited by age and fitness (generally ≤65–70 years)
Whole-brain radiation therapy (WBRT)	23.4–45 Gy (reduced-dose when used after chemotherapy)	R-MPV + reduced-dose WBRT (MSKCC); IELSG32	Effective consolidation; however, high risk of delayed neurotoxicity — leukoencephalopathy, cognitive decline, especially in patients >60 years
High-dose cytarabine	Cytarabine 3 g/m² × 4 doses (2 cycles)	Used in non-transplant eligible patients	Less toxic than ASCT; unclear if adequate for long-term control
No consolidation (maintenance MTX)	Ongoing HD-MTX cycles	Some older protocols	Avoids consolidation toxicity; higher relapse rates

WBRT-Related Neurotoxicity in PCNSL

WBRT causes severe delayed leukoencephalopathy in 25–50% of long-term survivors, particularly those >60 years old
Manifests 6 months to years after treatment: progressive cognitive decline, gait apraxia, urinary incontinence (resembling normal pressure hydrocephalus)
Imaging shows confluent periventricular white matter changes, cortical atrophy, ventriculomegaly
This devastating complication has driven the shift toward ASCT as the preferred consolidation strategy
Reduced-dose WBRT (23.4 Gy) after chemotherapy-induced CR may lower neurotoxicity risk, but long-term cognitive effects remain a concern
For elderly patients not eligible for ASCT, some centers avoid WBRT entirely and use chemotherapy alone or high-dose cytarabine consolidation

Treatment of Specific Populations

Elderly Patients (>70 years)

HD-MTX remains feasible and effective if renal function is adequate (GFR ≥50 mL/min)
ASCT is generally not feasible; consolidation options include high-dose cytarabine or maintenance lenalidomide
WBRT should be avoided in the elderly due to unacceptable neurotoxicity rates
Best supportive care with corticosteroids and WBRT may be appropriate for patients with poor performance status who cannot tolerate HD-MTX

HIV-Associated PCNSL

Institution of effective antiretroviral therapy (ART) is the first priority
Immune reconstitution alone may lead to tumor regression in some cases
HD-MTX-based regimens are used if feasible; drug interactions with ART require careful management
WBRT alone was the historical standard but is being replaced by combined modality therapy
Prognosis has improved dramatically in the ART era

Prognosis

IELSG Prognostic Score

Adverse Factor	Points
Age >60 years	1
ECOG performance status >1	1
Elevated serum LDH	1
Elevated CSF protein	1
Deep brain involvement (periventricular, basal ganglia, brainstem, cerebellum)	1

Risk groups: Low risk (0–1 points) — 2-year OS ~80%; intermediate risk (2–3 points) — 2-year OS ~50%; high risk (4–5 points) — 2-year OS ~15%.

Outcomes with Modern Treatment

Median overall survival: >5 years with HD-MTX-based induction followed by ASCT consolidation (compared with 12–18 months historically with WBRT alone)
Complete response rate: 40–60% with combination chemotherapy
5-year survival: 50–70% for younger, fit patients treated with optimal protocols; 20–30% for elderly patients
Long-term survivors require ongoing cognitive monitoring given the cumulative neurotoxicity of treatment

Relapsed/Refractory Disease

Treatment at Relapse

Scenario	Treatment Options	Notes
Relapse after >12 months, prior MTX response	Re-induction with HD-MTX-based regimen	Response to re-challenge likely if prior remission lasted >12 months
Refractory or early relapse (<12 months)	Ibrutinib, lenalidomide + rituximab, temozolomide, pemetrexed	Ibrutinib has ~50–70% ORR as single agent in relapsed PCNSL (MYD88/CD79B mutated)
ASCT-eligible at relapse	Salvage chemotherapy → ASCT	Option for patients who did not receive ASCT as initial consolidation
WBRT-naive	WBRT as salvage	Effective but with significant neurotoxicity; generally reserved for patients without better options

Emerging Therapies

Ibrutinib: Bruton tyrosine kinase (BTK) inhibitor; high CNS penetration; response rates of 50–70% in relapsed PCNSL; MYD88 L265P and CD79B mutations predict response; maintenance ibrutinib being explored
Lenalidomide: Immunomodulatory agent with CNS activity; used alone or in combination with rituximab; response rates ~35–65% in relapsed disease
Pomalidomide: Next-generation IMiD with good CNS penetration; under investigation
Tirabrutinib: Approved in Japan for relapsed PCNSL; other BTK inhibitors in clinical trials
CAR T-cell therapy: Anti-CD19 CAR T cells being investigated for relapsed PCNSL; early case series showing responses; concerns about neurotoxicity (ICANS) in an already-vulnerable CNS
Immune checkpoint inhibitors: Limited efficacy as single agents in PCNSL (unlike systemic DLBCL); 9p24.1/PD-L1 amplification may predict response

Secondary CNS Lymphoma

Secondary CNS lymphoma (SCNSL) refers to CNS involvement by systemic lymphoma and must be distinguished from PCNSL, as the treatment approach differs:

Feature	Primary CNS Lymphoma	Secondary CNS Lymphoma
Definition	Lymphoma confined to the CNS at diagnosis	Systemic lymphoma with secondary CNS spread
CNS involvement pattern	Parenchymal masses (most common)	Leptomeningeal disease (most common); parenchymal masses less frequent
High-risk histologies for SCNSL	—	Double-hit/triple-hit lymphoma (MYC + BCL2/BCL6), testicular DLBCL, intravascular lymphoma, Burkitt lymphoma
CNS prophylaxis	—	Intrathecal MTX or cytarabine; systemic HD-MTX incorporated into R-CHOP for high-risk patients
Treatment	HD-MTX-based regimen → consolidation	Treatment of systemic disease + CNS-directed therapy (HD-MTX, intrathecal chemo, radiation)

CNS Prophylaxis in High-Risk Systemic Lymphoma

Risk Factors for Secondary CNS Involvement

Histologic subtypes: Double-hit/triple-hit lymphoma (3–10% CNS risk), testicular DLBCL (15–30%), intravascular lymphoma, Burkitt lymphoma
CNS-IPI score: Validated tool incorporating age, LDH, ECOG, stage, extranodal sites, and kidney/adrenal involvement; high CNS-IPI (≥4) identifies patients at ~10% risk
Specific sites: Testicular, breast, adrenal, kidney, uterine involvement
Prophylaxis options: Intrathecal methotrexate (4–6 doses); systemic HD-MTX (3–3.5 g/m², 2–4 cycles) interdigitated with R-CHOP; the optimal prophylaxis strategy remains debated
Neither intrathecal nor systemic prophylaxis has been proven in randomized trials to prevent CNS relapse

Neurotoxicity Monitoring and Survivorship

Baseline neurocognitive assessment: Recommended before treatment; tests of memory, attention, executive function, and processing speed
Follow-up MRI: Every 3 months for the first 2 years, then every 6 months for years 3–5, then annually
Neurocognitive monitoring: Annual neuropsychological testing, especially after WBRT
Late effects: Treatment-related leukoencephalopathy (especially post-WBRT), persistent cognitive deficits, fatigue, mood disorders
Cognitive rehabilitation: May benefit survivors with treatment-related cognitive impairment

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