Incidence: 2% of glial tumors of ependymal origin; 0.5–1% of CNS tumors

Origin: neuroepithelial / endodermal

Histo: single layer of ciliated + columnar mucin-secreting epithelium; squamous cells of ependymal origin; tough fibrous capsule; filled with thick viscous mucus consisting of blood products, macrophages, cholesterol crystals, numerous metallic ions

Age: 5^th–6^th decade; M >F

• positional headaches (← transient obstruction secondary to ball-valve mechanism at foramen of Monro)
• hypogonadism, galactorrhea (for intra- / suprasellar location)
• change in mental status ± dementia (related to increased intracranial pressure); gait apraxia
• papilledema (may become medical emergency with acute herniation)

Location: inferior aspect of septum pellucidum protruding into anterior superior portion of 3^rd ventricle between columns of fornix; suprasellar cistern (rare)

Size: several mm to 3 cm

• ± sellar erosion
• 3^rd ventricular enlargement (to accommodate cyst anteriorly)
• asymmetric lateral ventricular enlargement (invariably)
• occasionally widens septum pellucidum
• may show enhancement of border ← draped choroid plexus / capsule

NECT:

• spherical iso- / (often) hyperattenuating lesion with smooth surface contour without enhancement
• fluid contents:
  1. in 20% similar to CSF (= isodense)
  2. in 80% mucinous fluid, proteinaceous debris, hemosiderin, desquamated cells (= hyperdense)

MR:

• variable SI lesion (dependent on mucus composition):
  • hyperintense on T1WI + FLAIR ← large protein molecules / paramagnetic effect of magnesium, copper, iron in cyst
  • iso- to hypointense on T2WI (most common)

Prognosis: asymptomatic and stable in 90%; 10% enlarge / cause hydrocephalus; with rapid enlargement → coma and death

DDx: meningioma, ependymoma of 3^rd ventricle (rare) with enhancement