====== Subependymoma Pathology ======

===== Macroscopic appearance =====

[[Subependymoma]]s are sharply demarcated [[nodule]]s, 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