Hi and welcome to HCM. Thanks for the query.
Ct is needed to verify exact size and position of cyst
and then further diagnostic and therapeutic procedures. If cysts is seen on xray already then i suppose it is big one. but it still doesnt means that surgery is neccesary. it must be removed if there is risk of hemorrhage or if this is symptomatic. since you have some pain then surgery can be considered. mostly such cysts are caused by trauma but som other causes are possible such as chronuc pancreatits or congenital cysts.
Wish you good health. Regards