This type of cyst is normally formed in utero.
Arachnoid cysts are benign cysts that occur in the cerebrospinal axis in relation to the arachnoid membrane and that do not communicate with the ventricular system. They usually contain clear, colorless fluid that is most likely normal cerebrospinal fluid; rarely, they contain xanthochromic fluid.
Most are developmental anomalies. A small number of arachnoid cysts are acquired, such as those occurring in association with neoplasms or those resulting from adhesions occurring in association with leptomeningitis, hemorrhage, or surgery. They constitute approximately 1% of intracranial masses; 50-60% occur in the middle cranial fossa. Cysts in the middle cranial fossa are found more frequently in males than in females; they occur predominantly on the left side. Most arise as developmental anomalies. A small number of arachnoid cysts are associated with neoplasms. (See the images below.)T2-weighted sagittal MRI image (see next image for axial view) of the brain in a 28-year-old woman with an incidental finding of a superior cerebellar cistern arachnoid cyst (arrow).This is exactly same spot that Jack has his cyst. We are getting a second opinon from a nuerologist.