Primary Central Nervous System Lymphoma (PCNSL) is an uncommon form of non-Hodgkin lymphoma restricted to the brain, spinal cord, eyes, or the leptomeninges, the membranes surrounding the brain. It accounts for approximately 3% of all primary brain tumors. Magnetic Resonance Imaging (MRI) is the primary imaging method for evaluating suspected cases, providing detailed images for initial detection and characterization.
The Purpose of MRI in PCNSL Diagnosis
Magnetic Resonance Imaging is the preferred tool for suspected PCNSL because it generates detailed images of the brain’s soft tissues without using ionizing radiation. It employs a magnetic field, radio waves, and computer processing to create cross-sectional images of the brain, revealing subtle abnormalities.
A standard procedure involves the intravenous injection of a gadolinium-based contrast agent. This substance travels through the bloodstream and accumulates where the blood-brain barrier (BBB) is compromised. The BBB is a protective network of blood vessels and tissue that is selective in allowing substances to pass from the blood into the brain.
In healthy individuals, the contrast agent cannot cross this barrier. However, PCNSL tumors disrupt the BBB, allowing gadolinium to leak into the tumor tissue. This leakage, known as contrast enhancement, causes the tumor to appear brightly on the MRI scan and provides a strong indication of an underlying pathology.
Characteristic MRI Findings of PCNSL
When examining MRI scans for PCNSL, radiologists look for a combination of features, with the lesion’s location being a primary clue. These tumors are frequently found in the periventricular regions (white matter adjacent to the ventricles), deep gray matter structures like the basal ganglia and thalamus, and the corpus callosum.
The most telling feature is the pattern of enhancement after a gadolinium-based contrast agent is administered. The tumors demonstrate strong, solid, and uniform enhancement, meaning the entire lesion lights up brightly and evenly, without the dark, non-enhancing areas often seen in other brain tumors. While most cases present as a single lesion, multiple lesions can be present at diagnosis.
Different MRI sequences provide unique information. On T2-weighted and FLAIR sequences, there is often surrounding edema, but less than one might expect for a lesion of its size. Diffusion-Weighted Imaging (DWI) is also informative, as PCNSL tumors are characteristically hypercellular, meaning they are densely packed with lymphoma cells. This high cellularity restricts the movement of water molecules within the tumor tissue. As a result, PCNSL lesions appear bright on DWI scans, a finding called restricted diffusion, which helps differentiate PCNSL from other brain lesions that are less cellular.
Distinguishing PCNSL from Other Brain Lesions
Differentiating PCNSL from other conditions that appear similar on an MRI is an important step. One of the most common mimics is glioblastoma, an aggressive primary brain cancer. While PCNSL shows solid, uniform enhancement, glioblastomas often display “ring enhancement,” a bright ring surrounding a central area of necrosis (dead tissue), which is less common in PCNSL.
Brain metastases, cancers that have spread to the brain from elsewhere, can also be confused with PCNSL. Metastases often present as multiple lesions at the junction between the brain’s gray and white matter. While PCNSL can also be multifocal, its location and homogenous enhancement pattern help point toward the correct diagnosis.
In patients with compromised immune systems, infections like toxoplasmosis can closely resemble PCNSL, also presenting with ring-enhancing lesions. This similarity can make it difficult to distinguish between the two based on imaging alone. Advanced MRI techniques measuring blood volume can also aid in differentiation, as lymphomas show lower relative cerebral blood volume than glioblastomas and metastases.
The Role of MRI After the Initial Scan
Although MRI findings can be highly suggestive of PCNSL, a definitive diagnosis requires a brain biopsy. During this procedure, a neurosurgeon obtains a small sample of tumor tissue for analysis. The initial MRI serves as a precise map, allowing the surgeon to accurately target the lesion and retrieve a sample while minimizing risks.
After a confirmed diagnosis, MRI is used for managing the disease and monitoring treatment response. PCNSL is treated with chemotherapy, and regular follow-up MRI scans assess the therapy’s effectiveness. These scans show whether the tumor is shrinking, remaining stable, or growing, allowing oncologists to make timely adjustments to the treatment plan.
MRI is also used for long-term surveillance after treatment is completed to detect any tumor recurrence. By comparing new scans to previous ones, doctors can identify new areas of growth early, which is a standard part of follow-up care for patients with PCNSL.