CNS Lymphoma: Imaging, Methotrexate, and CAR T-Cell Therapy

Central nervous system lymphoma, usually called CNS lymphoma or PCNSL (primary CNS lymphoma), is a rare and aggressive form of non-Hodgkin lymphoma that arises within the brain, spinal cord, cerebrospinal fluid, or eyes without evidence of disease elsewhere in the body. It accounts for a small fraction of all brain tumors, but its incidence has been climbing steadily for decades, and the disease poses unusual challenges in diagnosis and treatment because the blood-brain barrier shields it from many standard cancer therapies. With prompt, appropriate treatment, roughly 30 percent of patients survive five years, a figure that has improved considerably in recent decades but still lags behind many other lymphomas.

Who Gets It and Why the Numbers Are Rising

CNS lymphoma was once considered extremely rare. Between 1975 and 2017, the incidence rate in the United States rose from about 0.1 per 100,000 people to 0.5 per 100,000, a fivefold increase with an average annual percentage change of about 5.3 percent.¹ That upward trend has two distinct stories behind it. During the late 1980s and early 1990s, the HIV/AIDS epidemic drove a surge of CNS lymphoma in younger men with severely compromised immune systems. As antiretroviral therapy became widely available, that wave receded sharply. The proportion of CNS lymphoma cases linked to HIV fell from about 64 percent in the early 1990s to roughly 13 percent by the late 2000s.²

What replaced that HIV-driven wave is a slower, sustained rise among older adults with otherwise healthy immune systems. Among people 65 and older, incidence rates have been climbing by roughly 1.5 to 2 percent per year, and adults 75 and older have experienced the most dramatic increase over time.³ The disease is also somewhat more common in men than women across all age groups. Why immunocompetent elderly people are increasingly developing this cancer remains an open question, though age-related immune decline is the leading hypothesis.

The Immune System Connection

Immune status is the single strongest known risk factor for CNS lymphoma. People with severely weakened immune systems, whether from HIV, organ transplantation, or inherited immune disorders, develop PCNSL at rates far higher than the general population. In these patients, the disease is almost always driven by the Epstein-Barr virus (EBV), the same virus responsible for infectious mononucleosis. Studies of AIDS-related CNS lymphoma have found EBV in essentially 100 percent of tumor cells, a far higher association than in systemic lymphomas, where EBV appears in only 30 to 50 percent of cases.⁴ That near-universal EBV presence suggests the virus plays a direct role in causing the tumor when immune surveillance collapses.

Immunodeficiency-associated CNS lymphoma is now understood to be biologically distinct from the kind that arises in immunocompetent patients. Tumors in immunosuppressed patients are typically EBV-positive, carry different genetic alterations, and have a worse prognosis.⁵ In contrast, the majority of CNS lymphomas arising in immunocompetent people are EBV-negative and driven by a different set of molecular events. A small subset of elderly patients with no known immune deficiency still develop EBV-positive CNS lymphoma, likely reflecting age-related immune decline (immunosenescence) that allows the virus to reactivate. One screening study found that about 4 percent of primary CNS diffuse large B-cell lymphomas in a series of 134 cases were EBV-positive in patients 65 or older who had no history of immune deficiency.⁶

Genetic Fingerprints of the Tumor

In immunocompetent patients, CNS lymphoma has a strikingly consistent genetic profile. The vast majority of tumors carry mutations that chronically activate a signaling pathway called NF-κB, which normally helps immune cells survive and multiply. One study of 71 tumors found that 96 percent had mutations in the NF-κB gene network, with the two most commonly affected genes being CD79B (mutated in 83 percent) and MYD88 (mutated in 76 percent).⁷ These two mutations frequently appear together in the same tumor, which is unusual in cancer biology and suggests a cooperating relationship. A second study found MYD88 mutations in about 69 percent and CD79B mutations in roughly 32 percent of cases, with the most common specific change being the MYD88 L265P hotspot mutation.⁸

These genetic features matter for treatment. The MYD88 and CD79B mutations activate a protein called Bruton tyrosine kinase (BTK), which has become a drug target. The high frequency and consistency of these mutations also help distinguish CNS lymphoma from other brain tumors, a distinction that can guide therapy when biopsy results are ambiguous.

How It Shows Up on Imaging

On MRI, CNS lymphoma typically appears as a solitary or multiple lesion that lights up uniformly with contrast dye, looks darker than surrounding brain tissue on certain sequences, and shows restricted water diffusion, a sign of densely packed cells. These imaging features overlap considerably with other brain tumors, particularly glioblastoma, which is far more common. Several imaging techniques can help tell them apart. CNS lymphomas tend to have lower blood volume on perfusion imaging and lower diffusion values than glioblastomas. Combining diffusion, perfusion, and susceptibility-weighted imaging was able to correctly predict the diagnosis in 95 percent of CNS lymphoma cases and 96 percent of atypical glioblastoma cases in one study.⁹

Even with advanced imaging, the distinction is not always straightforward. Other conditions including neurosarcoidosis, tumefactive multiple sclerosis, and certain infections can mimic CNS lymphoma on scans. Tissue diagnosis through biopsy remains the standard, but there is an important pitfall in how that biopsy happens.

The Steroid Trap

One of the most clinically significant diagnostic challenges in CNS lymphoma involves corticosteroids like dexamethasone. When a patient shows up with a large brain mass causing swelling and neurological symptoms, the instinct is to give steroids to reduce the pressure. The problem is that CNS lymphoma is exquisitely sensitive to steroids, which can kill enough tumor cells to make the mass shrink or even temporarily disappear on imaging before a biopsy can be performed. When a biopsy is then taken, the pathologist may find only inflammation and dead cells, not enough viable lymphoma to make a definitive diagnosis.

A systematic review and meta-analysis found that patients who received steroids before a stereotactic brain biopsy had three times the risk of getting a nondiagnostic result compared to those who did not.¹⁰ This creates a frustrating situation: the patient needs symptom relief, but the medication that provides relief can delay or prevent the diagnosis that would guide proper treatment. Current practice guidelines recommend withholding steroids whenever safely possible until a tissue diagnosis has been obtained. When steroids have already been given and the biopsy is nondiagnostic, a repeat biopsy after steroid withdrawal may be necessary.

Eye Involvement

The eyes are technically part of the central nervous system, and CNS lymphoma has a particular affinity for the vitreous humor and retina. This ocular form, sometimes called vitreoretinal lymphoma, can cause blurred vision, floaters, or painless visual loss. It may appear before, alongside, or after brain disease. Diagnosing it is notoriously difficult. In one series of 27 patients with vitreoretinal lymphoma, only 37 percent received a definitive diagnosis on the first procedure, and 63 percent had at least one false-negative result before lymphoma was confirmed.¹¹ Diagnosis often requires a surgical vitrectomy (removing some of the gel inside the eye) to obtain enough cells for analysis, and even then the yield can be low because the malignant cells are fragile and scarce.

Because of this eye-brain connection, all patients diagnosed with CNS lymphoma undergo a detailed eye examination as part of staging. Conversely, patients diagnosed with vitreoretinal lymphoma receive brain and spinal imaging and a lumbar puncture to check for CNS involvement.

Treatment Backbone: High-Dose Methotrexate

The standard of care for CNS lymphoma revolves around high-dose intravenous methotrexate, a chemotherapy drug that, at sufficient doses, crosses the blood-brain barrier. This is what distinguishes treatment from the regimens used in systemic lymphoma, which typically rely on drugs that do not penetrate the brain well. Methotrexate alone produces complete responses in about 36 to 41 percent of patients, but adding rituximab (an antibody that targets B cells) dramatically improves results. In one retrospective study, complete response rates doubled from 36 percent with methotrexate alone to 73 percent with methotrexate plus rituximab, and median time to disease progression jumped from about 4.5 months to nearly 27 months.¹²

Multi-drug regimens tend to do better still. A meta-analysis of clinical trials found that three- and four-drug combinations based on high-dose methotrexate outperformed methotrexate alone, with the MPV regimen (methotrexate, procarbazine, and vincristine) achieving pooled complete response rates of about 58 to 63 percent depending on whether rituximab was included.¹³ Treatment is typically delivered in an induction phase to achieve remission, followed by some form of consolidation to reduce the risk of relapse.

After Remission: Choosing a Consolidation Strategy

The choice of consolidation therapy has been one of the most debated questions in the field. The two main options are whole-brain radiation therapy (WBRT) and high-dose chemotherapy followed by autologous stem-cell transplant (ASCT). For years, whole-brain radiation was the default, but concerns about delayed neurotoxicity, especially in older patients, have pushed the field strongly toward transplant when feasible.

The randomized PRECIS trial provided long-term evidence for this shift. Among patients 60 and younger, eight-year event-free survival was 67 percent in the transplant arm compared with 39 percent in the whole-brain radiation arm, driven largely by a much lower relapse rate after transplant. However, eight-year overall survival was similar between the groups (69 percent vs. 65 percent), partly because patients who relapsed after radiation could be salvaged with other treatments.¹⁴ The investigators concluded that 40 Gy whole-brain radiation should be avoided in first-line treatment because of its neurotoxicity and inferior relapse prevention.

A larger, more recent phase 3 trial (MATRix) reinforced transplant’s advantage. Patients who received thiotepa-based high-dose chemotherapy followed by transplant had a three-year progression-free survival of 78 percent, compared with 51 percent in a non-myeloablative chemoimmunotherapy arm.¹⁵ A Mayo Clinic series found roughly two-thirds of transplanted patients remained progression-free at two years, regardless of the specific conditioning regimen used.¹⁶

Whole-brain radiation has not been abandoned entirely. It is still used as salvage therapy when lymphoma recurs after chemotherapy, and in that setting it remains effective.¹⁷ Reserving radiation for relapse rather than using it upfront is considered a reasonable strategy to delay the risk of cognitive damage.

Cognitive Consequences and Long-Term Quality of Life

The brain is both the battlefield and the organ that defines a person’s identity, which makes cognitive toxicity one of the most important quality-of-life concerns in CNS lymphoma. Survivors who received whole-brain radiation scored significantly lower on tests of attention, executive function, and motor skills compared with those treated without radiation.¹⁸ This radiation-related cognitive decline tends to worsen over time and can be devastating in older patients, sometimes mimicking dementia.

Chemotherapy-only approaches are not without cognitive cost either. The disease itself damages brain tissue, and high-dose methotrexate can cause white-matter changes. But the magnitude of cognitive decline is generally less severe than what follows whole-brain radiation, which is the practical reason the field has moved toward transplant-based consolidation. For patients who achieve long-term remission, cognitive rehabilitation and neuropsychological monitoring are increasingly recognized as important parts of survivorship care.

Treating Older and Frail Patients

Stem-cell transplant is an intense procedure that many older or medically frail patients cannot tolerate. Since CNS lymphoma peaks in people over 65, a substantial proportion of newly diagnosed patients are not transplant candidates. This creates a treatment gap: the best consolidation strategy is off the table for the patients most likely to develop the disease.

For elderly patients, modified chemotherapy regimens aim to balance efficacy against toxicity. One large study (PRIMAIN) evaluated a regimen of rituximab, methotrexate, and procarbazine with or without lomustine in elderly patients. Dropping lomustine produced similar progression-free and overall survival with less toxicity, leading the investigators to recommend the simplified regimen as a benchmark for future trials.¹⁹ Even with these adjusted regimens, treatment decisions in the elderly require careful assessment of kidney function (methotrexate is cleared by the kidneys), overall fitness, and cognitive baseline.

Newer Approaches: Targeted Drugs and CAR T-Cells

The consistent presence of MYD88 and CD79B mutations in CNS lymphoma has made it a promising target for drugs that block the downstream protein BTK. Ibrutinib, the first widely available BTK inhibitor, crosses the blood-brain barrier and reaches measurable concentrations in cerebrospinal fluid. In an early study, CSF drug levels increased with higher oral doses and with continued therapy over time.²⁰ BTK inhibitors are now being tested in combination with chemotherapy in frontline and relapsed settings, and several trials are ongoing.

CAR T-cell therapy, in which a patient’s own immune cells are engineered to recognize and kill lymphoma cells, has generated considerable excitement. A meta-analysis of 128 patients with primary or secondary CNS lymphoma treated with CAR T-cells found overall response rates of about 64 percent in primary CNS lymphoma, with 56 percent achieving a complete response, a striking result in patients who had already failed multiple prior treatments.²¹ Across several studies, one-year progression-free survival reached 40 to 60 percent.²²

The concern, predictably, is toxicity. Cytokine release syndrome (a systemic inflammatory response) occurred in about 70 percent of patients, though severe cases were uncommon. Neurotoxicity was the bigger worry: roughly half of patients experienced neurological side effects of some kind, and about one in five to one in four had serious (grade 3-4) neurological events including cognitive disturbances, balance problems, and seizures.²³ These rates are higher than what is seen when CAR T-cells are used for lymphoma elsewhere in the body, which makes sense given that the engineered cells are attacking targets inside the brain. Still, most events resolved, and researchers consider the toxicity profile manageable enough to justify continued development.

Monitoring With Liquid Biopsy

One emerging tool that could change how the disease is tracked is the detection of tumor DNA floating freely in cerebrospinal fluid, known as circulating tumor DNA (ctDNA). Unlike imaging, which can take weeks to show changes, ctDNA levels in the spinal fluid appear to rise and fall in near-real time with tumor activity. In one study, CSF ctDNA levels dropped sharply after treatment and rose again before clinical relapse was visible on MRI or detectable by conventional cell-counting methods. In at least two patients, ctDNA reappeared in the spinal fluid months before imaging confirmed relapse.²⁴ Importantly, plasma ctDNA (from a simple blood draw) was not useful for monitoring CNS-confined disease, since the tumor DNA stayed behind the blood-brain barrier.

This finding has practical implications. If validated in larger studies, serial CSF ctDNA testing could allow clinicians to detect relapse early enough to intervene before a patient develops new symptoms or visible tumor on scans. It could also help confirm whether a treatment is working without waiting weeks for the next MRI. For now, the technology remains investigational, but it is among the more promising developments in a disease that has historically been difficult to monitor.