Pineal cysts are fluid filled sacs occurring in the pineal gland. They are are generally considered benign if they are less than 5 mm. In fact, these small (less than 5 mm) cysts are relatively common and found in over 25% of sampled cadavers. These small pineal cysts rarely cause symptoms.
With your pineal cyst measuring 9 mm, it would be considered a large pineal cyst which may be associated with a variety of symptoms such as headaches, vision changes, and abnormal sleep/wake patterns. These large cysts may also be associated with brain abnormalities such as seizures and migraines. The symptoms of large cysts are mainly due to compression of adjacent structures.
Surgery for pineal cysts are mainly indicated for patients who’s symptoms are severely debilitating. In your case, the pineal gland mass is described as a cyst which indicates that its benign; it would be warranted to discuss the risks/benefits of any surgery with a neurosurgeon.