CNS Lymphoma
- Primary CNS lymphoma is an aggressive brain tumor that loves the deep, central real estate — periventricular, basal ganglia, corpus callosum — and often hugs the ventricular surface.
- It's so densely packed with cells that it looks dense on CT, dark (restricted) on diffusion, and usually enhances solidly and homogeneously in someone with a normal immune system.
- It is famously steroid-sensitive: a dose of corticosteroids can make the tumor melt off the scan before biopsy, which is a disaster for the pathologist. Don't pre-treat if lymphoma is on the table.
- In immunocompromised patients (think advanced HIV) the rules change — it gets necrotic and ring-enhancing, and now looks a lot like toxoplasmosis.
Most brain tumors are show-offs. They grow a fat rind of dying tissue, ooze a halo of edema, and ring-enhance like a glazed donut. Primary CNS lymphoma (PCNSL) is the opposite kind of villain: lean, dense, and disciplined. It's a non-Hodgkin lymphoma (almost always B-cell) that sets up shop inside the brain itself, with no lymphoma anywhere else in the body. And it has a very particular set of habits that, once you know them, make it one of the more satisfying calls in neuroradiology.
The one-sentence version: too many cells, not enough space
Here's the whole personality of this tumor in one idea. Lymphoma cells are small and they pack together like commuters on a rush-hour train — shoulder to shoulder, almost no room between them. Radiology sees cell density. When tissue is that crowded, three things happen at once, and they're really all the same thing wearing different costumes:
- On CT, all those nuclei make the mass look dense (hyperattenuating) compared to normal brain, even before contrast.
- On diffusion imaging, water molecules can't wiggle freely between cells, so they get stuck — restricted diffusion, bright on DWI and dark on the ADC map.
- On T2, the same crowding can make the mass look surprisingly low in signal for a tumor.
If you remember nothing else, remember "dense." It explains almost every imaging feature on this page.
Where it likes to live
PCNSL is a homebody, and it picks the deep, central neighborhoods. The classic locations are periventricular white matter, the basal ganglia and thalami, and the corpus callosum — it loves to cross the midline through the callosum, the same way glioblastoma does, so "butterfly lesion" is on both their resumes. It also has a habit of contacting the ependymal (ventricular) surface. A deep, midline-crossing, ventricle-hugging mass should put lymphoma high on your list.
The enhancement tell (in a normal immune system)
In an immunocompetent patient, give contrast and PCNSL typically lights up avidly, solidly, and homogeneously — a filled-in lesion, not a ring. That solid pattern, combined with the restricted diffusion and the deep location, is the money combination.
Solid homogeneous enhancement + restricted diffusion + deep periventricular location = think CNS lymphoma until proven otherwise.
Why no ring? Because the tumor isn't outgrowing its blood supply and rotting in the center — it's just a dense, well-perfused wall of cells. (That changes in immunocompromised patients; see below.) Compared to a brain metastasis, lymphoma tends to be more central and more solid, and metastases are usually multiple and sit at the gray-white junction where tiny tumor emboli get stuck.
The vanishing-act pitfall (the most important thing here)
PCNSL is exquisitely steroid-sensitive. Give corticosteroids — which everyone reaches for to treat brain edema — and the tumor can shrink or even disappear from the scan within days. It's been nicknamed the "ghost tumor" or "melting tumor" for exactly this. If you suspect lymphoma, the message to the team is: hold the steroids and get tissue first. A treated, melted lesion can leave the pathologist with nothing diagnostic to biopsy, and the diagnosis gets lost in the fog.
This is one of those rare situations where the radiologist's report genuinely changes the patient's path before anyone picks up a scalpel.
When the immune system is down, the disease changes clothes
Everything above describes the immunocompetent patient. In someone immunocompromised — classically advanced HIV/AIDS — lymphoma behaves differently: it's more likely to be necrotic and ring-enhancing, and it may be multifocal and hemorrhagic. Now it looks much less tidy, and its main rival on the differential is toxoplasmosis, the other big enhancing brain lesion in HIV (covered with the other CNS infections).
| Feature | CNS lymphoma (immunocompromised) | Toxoplasmosis |
|---|---|---|
| Number | Often solitary, but frequently multifocal in this setting | Usually multiple |
| Location | Often periventricular / deep | Basal ganglia, gray-white junction |
| Thallium-201 / FDG-PET | Increased uptake (it's a tumor) | No uptake (it's infection) |
| Response to anti-toxo therapy | No improvement | Improves over ~2 weeks |
A practical real-world tiebreaker: empiric anti-toxoplasmosis treatment for a couple of weeks. If the lesion shrinks, it was probably toxo. If it sits there unimpressed, lymphoma climbs the list and biopsy follows. Nuclear imaging (FDG-PET or thallium SPECT) helps too — tumor is metabolically hungry, infection isn't.
The takeaway
Primary CNS lymphoma is the dense, disciplined tumor that lives deep, crosses the corpus callosum, hugs the ventricles, restricts diffusion, and — in a normal immune system — enhances solidly rather than in a ring. The single fact that can change a patient's outcome is the steroid sensitivity: suspect it, and ask the team to hold the steroids until there's tissue. Everything else flows from "too many cells, not enough room."