Primary CNS lymphoma (PCNSL)
Primary CNS lymphoma frequently involves the deep gray matter (thalamus and basal ganglia in 15% of cases). About 10% of tumors are located in the posterior cranial fossa, showing a midline predilection.
Imaging features:
- Unenhanced CT: Tumor masses are hyperdense in 70% of cases. Calcifications are uncommon, while mass effect and vasogenic edema are relatively mild.
- MRI: Lesions are usually isointense on T1- and T2-weighted images.
- Contrast enhancement: Both on CT and MRI (following gadolinium administration), marked homogenous contrast enhancement is observed. An exception is patients with AIDS, who may present with atypical ring-like enhancement.
- MRI (DWI): Restriction of diffusion within the mass is characteristic due to high tumor cellularity.
In immunocompetent patients, multifocal growth occurs in 15% of cases, whereas in the setting of AIDS, this rate increases to 60%. To differentiate lymphoma from infections (e.g., toxoplasmosis) in HIV-positive patients, FDG-PET/CT is used.
Secondary CNS involvement
Secondary CNS involvement is extremely rare in Hodgkin lymphoma, but occurs in 10–15% of patients with non-Hodgkin lymphomas (NHL) as the disease progresses.
High-risk groups include stage IV patients (especially with involvement of the orbits, paranasal sinuses, testes, or ovaries), those with aggressive histological variants (immunoblastic, lymphoblastic), and those with more than one extranodal site initially identified.
Location and diagnosis: Secondary lymphoma has a predilection for extracerebral spaces: subarachnoid, subdural, and epidural (including at the spinal level). Clinically, this often manifests as cranial nerve palsies.
For detecting leptomeningeal and subdural involvement, contrast-enhanced MRI is significantly superior to CT. The lesions appear as enhancing plaques along the meninges of the skull base and convexity.
Spinal involvement
Spinal involvement requires special attention, as CT may fail to show minimal changes. MRI is the absolute modality of choice.
- Leptomeningeal involvement: Detected on contrast-enhanced MRI, although the risk of false-negative results is higher here than in meningeal carcinomatosis.
- Epidural involvement: May lead to spinal cord compression or cauda equina syndrome. Most commonly, the tumor invades the epidural space from paravertebral lymph nodes through the intervertebral foramen, forming a classic dumb-bell tumour.
- The dura mater itself serves as a robust barrier, thereby restricting intrathecal spread.
Briefly: answers to topical questions
Which histological subtypes of non-Hodgkin lymphoma most frequently metastasize to the CNS?
High-risk groups for secondary CNS involvement include patients with high-grade histological variants of NHL, particularly lymphoblastic and immunoblastic lymphoma.
What clinical symptoms characterize secondary meningeal involvement in lymphoma?
Secondary meningeal involvement in lymphoma often presents with cranial nerve palsies. Involvement of the spinal epidural space can lead to spinal cord compression and cauda equina syndrome.
PCNSL = Dense (hyperdense on CT), Devoid of calcifications, Completely enhancing, Cellularly rich (restricted diffusion on MRI).
Class and exam questions
What is the difference in contrast enhancement of PCNSL between immunocompetent patients and AIDS patients?
Immunocompetent patients typically show homogenous contrast enhancement. Patients with AIDS may exhibit atypical ring-like enhancement.
Why is restricted diffusion observed on MRI in CNS lymphoma?
This is due to the very high cellularity of the tumor mass, which restricts the movement of water molecules.
Which diagnostic imaging modality is preferred for detecting secondary CNS involvement?
Contrast-enhanced MRI (gadolinium). It is significantly superior to CT in visualizing meningeal involvement.
How does a dumb-bell tumor form in spinal lymphoma?
It forms when paravertebral tumor masses extend epidurally into the spinal canal through the intervertebral foramen.
What else to study on this topic
- Diagnostic imaging of orbital lymphoma and differential diagnosis
- Features of Waldeyer's ring lymphoma
- Radiological presentation of paranasal sinus involvement in NHL
- Specifics of salivary gland involvement in MALT lymphoma
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