A review of local cases demonstrating classical findings of PCNSL and its main differentials, which include malignancy, demyelination as well as other causes of infection, was carried out in order to highlight the typical findings which should prompt consideration of the diagnosis in the immunocompetent patient.

Primary CNS Lymphoma

Commonly supratentorial in origin, characteristic radiological findings are of a mass or multiple masses usually in contact with the subependymal surface.  These are classically hyperattenuating on unenhanced CT (figure 1) with MRI demonstrating low T1 and variable T2 signal with restricted diffusion and avid homogenous enhancement of the lesions.  Involvement of the corpus callosum with crossing of the midline is not infrequently seen (figures 2-4).  Surrounding oedema is usually mild and haemorrhage and calcification are rare.[5-9]  Follow up imaging in the same patient demonstrates resolution of the avidly enhancing periventricular masses after treatment (figure 5).

Fig. 1: PCNSL - Axial pre-contrast CT images (A) show hyperattenuating periventricular masses crossing the midline with surrounding oedema. Post-contrast CT images (B) demonstrate avid homogeneous enhancement of the lesions.

Fig. 2: PCNSL - Axial T2 (A) and axial FLAIR (B) images demonstrate periventricular masses of intensity similar to that of grey matter with surrounding vasogenic oedema.

Fig. 3: PCNSL - Axial DWI (A) and corresponding ADC (B) images showing restricted diffusion within the lesions.

Fig. 4: PCNSL - Axial T1 pre-contrast (A), axial (B) and sagittal (C) post contrast T1 images show avid homogeneous enhancement of the masses.

Fig. 5: PCNSL - Follow up scans from the same patient in figures 1-4 after treatment. Axial pre-contrast CT (A), DWI (B) and post-contrast T1 images demonstrate resolution of the masses.

Butterfly Glioma

Butterfly gliomas are high grade astrocytomas, usually glioblastoma multiforme, which cross the midline via the corpus callosum.  CT typically demonstrates a hypo to isoattenuating lesion with an irregular hypoattenuating centre representing necrosis.  Surrounding vasogenic oedema is typically pronounced and haemorrhage is occasionally encountered (figure 6).  Calcification is a rare feature.  MRI features consist of a mass demonstrating hypointensity on T1 weighted imaging and hyperintensity on T2WI with central heterogeneity accounting for intratumoral necrosis and haemorrhage.  Solid components of the tumor show elevated signal on DWI owing to increased cellularity (figure 7).  Enhancement is variable but almost always encountered, typically peripheral and nodular surrounding areas of necrosis (figure 8).[10]      

Fig. 6: Butterfly Glioma - Axial pre-contrast CT images showing a heterogeneous lesion involving both hemispheres and crossing the splenium of the corpus callosum. A hyperattenuating region in the left hemisphere represents acute haemorrhage.

Fig. 7: Butterfly Glioma - Axial FLAIR (A) and axial T2 (B) images showing a heterogeneous lesion with areas of central necrosis and surrounding vasogenic oedema involving the splenium of the corpus callosum. Axial DWI (C) demonstrates mild increased signal in the peripheral solid components.

Fig. 8: Butterfly Glioma - Pre-contrast coronal T1 (A), post-contrast coronal (B) and axial (C) images showing heterogeneous peripheral and nodular enhancement of the lesion surrounding areas of central necrosis.

Tumefactive Demyelination

Tumefactive demyelination is a locally aggressive form of demyelination, usually manifesting as a solitary lesion greater than 2 cm that may mimic a neoplasm on imaging.  CT shows a hypoattenuating lesion (figure 9) whilst MRI demonstrates a hyperintense T2 lesion with surrounding oedema and minimal mass effect.  Increased signal on DWI may be encountered (figure 10).   About half of tumefactive demyelinating lesions demonstrate contrast enhancement usually in the form on an open ring (figure 11).[11, 12]

Fig. 9: Tumefactive Demyelination - Axial unenhanced CT images showing hypoattenuation in the left periventricular region with little mass effect.

Fig. 10: Tumefactive demyelination - Axial T2 (A) and axial FLAIR (B) images showing 2 hyperintense lesions in the left periventricular region with surrounding oedema and paucity of mass effect. Axial DWI (C) demonstrates mildly increased peripheral signal.

Fig. 11: Tumefactive Demyelination - Coronal pre-contrast T1 (A), coronal (B) and axial (C) post-contrast images demonstrating characteristic open ring enhancement.

Cerebral Abscess

Cerebral abscesses result from pathogens growing within the brain parenchyma, initially as a cerebritis and then eventually demarcating into a cerebral abscess.  Typical findings are of a ring enhancing CT hypoattenuating (figure 12) and MRI hypointense T1 and hyperintense T2 lesion (figure 13).  DWI shows central restricted diffusion (figure 14) owing to a matrix of proteins, cellular debris, and bacteria in high-viscosity pus.[13]

Fig. 12: Cerebral Abscess - Axial pre- (A) and post-contrast (B) CT images showing a ring enhancing lesion in the left frontal lobe with surrounding vasogenic oedema.

Fig. 13: Cerebral Abscess - Pre-contrast coronal T1 (A), post contrast coronal (B) and axial (C) T1 images demonstrating ring enhancement and surrounding vasogenic oedema.

Fig. 14: Cerebral Abscess - Axial FLAIR (A) demonstrating a central area of hypointensity and DWI (B) showing marked characteristic central restricted diffusion.