A pineal glan cyst is a cyst in the pineal gland, a small endocrine gland in the brain.. Statisically, these fluid-filled bodies appear on 1-4% of magnetic resonance imaging (MRI) brain scans. They are more frequent at death, seen in 21-41% of autopsies, though researchers are not sure why. They are in most cases asymptomatic, and it is rare for a pineal cyst to cause symptoms. If they do, these would most likely be headache, visual disturbances, or hydrocephalus, which can be extremely dangerous if left untreated. Pineal cysts are not cancer and most of the time they do not enlarge over 2 centimeters.