There are four types of brain cysts, which include arachnoid, colloid, dermoid and epidermoid cysts, according to the American Brain Tumor Association. A cyst is a sac-like, fluid-filled pocket of tissue, explains WebMD.
Brain cysts are not tumors because they are not formed from brain tissue, explains the American Brain Tumor Association. Cysts that arise in parts of the brain that control important life functions require surgery, but most brain cysts are benign.
Dermoid and epidermoid cysts most often affect the skin rather than the brain, notes ABTA. Dermoid cysts, however, sometimes occur in the rear of the brain, while epidermoid cysts arise near the brain stem. Both of these cysts form early in fetal development, although symptoms do not manifest until years later. Fluid-filled arachnoid cysts form in the layers of tissue surrounding the outside of the brain. If arachnoid cysts are small, little treatment is necessary, and it most often involves observation to determine if the cyst is growing.
The American Brain Tumor Association cites no reports of malignant colloid cysts. Colloid cysts occur in the region of the brain near the cerebellum and probably begin during embryonic development. They are named for colloid, the thick fluid that forms inside the cysts. Surgery on colloid cysts is difficult, but often necessary given the affected area.