An arachnoid cyst of 5cm in an 8 year old child does not require treatment if it is not causing significant symptoms, only regular review is needed. Arachnoid cyst is a congenital disease, which is caused by congenital arachnoid development abnormality. Arachnoid cysts are not tumors and have no risk of malignancy. Generally speaking, arachnoid cysts do not have any clinical symptoms and do not affect the normal development of children, so they do not need treatment, and only need to be reviewed regularly, usually once a year to do a cranial magnetic resonance. If the arachnoid cyst is too large or in a special location that compresses normal brain tissue and nerves, causing symptoms such as headaches and persistent seizures, surgical treatment is required. Although arachnoid cysts themselves do not affect the normal development of children, symptoms caused by arachnoid cysts such as persistent epilepsy can affect a child’s intellectual development, so it is important to treat arachnoid cysts promptly when they become symptomatic. If necessary, a specialist is recommended to evaluate the condition.