Choroid plexus cyst
From wikipedia: "choroid_plexus_cyst" (cpcs) are cysts that occur within choroid
plexus of the brain.
The brain contains pockets or spaces called ventricles with a spongy layer of cells and blood vessels called the choroid plexus...Cpcs can form within this structure and come from fluid trapped within this spongy layer of cells...It is believed that many adults have one or more tiny cpcs.".