Myxoid glioneuronal tumor

Myxoid glioneuronal tumor is a rare and low-grade tumor (WHO grade 1) that usually involve the septum pellucidum, corpus callosum, subcallosal area, periventricular white matter and septal nuclei ¹. 

Terminology

The histologic features of this tumor are similar to dysembryoplastic neuroepithelial tumor (DNET) and therefore they were formerly called "DNET of the septum pellucidum" ².

In 2020, the cIMPACT-NOW Working Committee considered myxoid glioneuronal tumor to represent a distinct entity based on genetic mutations, imaging features and anatomic location; consequently naming it "myxoid glioneuronal tumor, PDGFRA p.K385- mutant" ³ and it is included in the 5^th Edition (2021)  WHO brain tumor classification ⁵.

Epidemiology

It often occurs in children and young adults. It represents about 2% of all brain tumors ³.

Pathology

Although it has been designated as a WHO grade 1 tumor in the current WHO brain tumor classification, myxoid glioneuronal tumor tends to recur, progress or disseminate in the ventricles. They may cause critical mass and compressive effect with obstructive hydrocephalus ¹.

Location

• septum pellucidum
• corpus callosum
• subcallosal area
• periventricular white matter
• septal nuclei

Microscopic appearance

Myxoid glioneuronal tumor demonstrates a low-grade proliferation of oligodendrocyte-like tumor cells and a prominent myxoid/mucin-rich stroma. It presents a fine capillary network reminiscent of DNET or oligodendroglioma. It may show neurocytic rosettes ¹.

Immunophenotype

• OLIG2, SOX10, GFAP: positive

Myxoid glioneuronal tumor is negative to BRAF and FGFR1 mutations or rearrangements, which distinguish DNETs, rosette-forming glioneuronal tumors (RGNTs) and other low-grade neuroepithelial tumors. Ki-67 index ranges from 1% to 4% ¹.

Radiographic features

This tumor has a well-defined lobulated mass appearance. 

MRI 

• T1: hypointense
• T2: hyperintense
• FLAIR: ​​bright rim sign, like DNETs ⁴
• T1 C+ (Gd): no enhancement

Treatment and prognosis

Data regarding prognosis for myxoid glioneuronal tumor are limited given the rarity of this entity. Surgical resection is often curative, although cases of recurrence, progression and intraventricular dissemination have been described. In these cases a second resection was frequently necessary. Only seldomly have radiotherapy and chemotherapy been used ¹.

Differential diagnosis

General imaging differential considerations include:

• colloid cyst
• central neurocytoma
• subependymoma