Subependymoma Pathology
Macroscopic appearance
Subependymomas are sharply demarcated nodules, usually no more than 2 cm in diameter, arising from the ependyma by a narrow pedicle
Size is the most important distinguishing feature compared to subependymal giant cell astrocytoma.
Microscopic appearance
The histopathology of subependymomas is distinct comprising of a tumor arising from the subependymal glial layer with low cellularity and no high-grade features (no mitoses, Ki-67/MIBI index <1.5%, no necrosis). These lesions are hypovascular. Loose perivascular pseudorosettes are occasionally seen. They are a WHO grade 1 tumor (see WHO classification of CNS tumors).
Occasionally foci of cellular ependymoma are seen, although the effect on clinical behavior is unclear
They are graded according to the ependymoma component and not surprisingly behave similarly to the higher grade (ependymoma) component.
Immunophenotype
Cells express GFAP 4-6,8. Unlike ependymomas, EMA is usually negative.
Radiographic features
Subependymomas are most commonly seen in the fourth ventricle, but can arise anywhere where there is ependyma. They are therefore in the differential for other intraventricular masses. Distribution in the ventricular system is as follows 6,8:
fourth ventricle: 50-60%
lateral ventricles (usually frontal horns): 30-40%
third ventricle: rare
central canal of the spinal cord: rare
They are usually small, typically less than 1-2 cm in size.
CT
Isodense to somewhat hypodense intraventricular mass compared to adjacent brain, which does not usually enhance. If large it may have cystic or even calcific components (seen in up to half of cases 3). Surrounding vasogenic edema is usually absent.
MRI
T1 iso-hypointense to white matter generally homogeneous but may be heterogeneous in larger lesions
T2 hyperintense to adjacent white and grey matter again heterogeneity may be seen in larger lesions, occasionally with susceptibility related signal dropout due to calcifications no adjacent parenchymal edema (as no brain invasion is present)
T1 C+ (Gd) usually no enhancement, although at times may demonstrate mild enhancement
DSA-angiography
As expected from the histology, subependymomas show no or little vascularity