Accounting interim data delta

Hi, we have a radius server installed collecting accounting data from a Cellular network. Currently we receive the Interim update every 30 minutes, and it updates the radacct table correctly . What we need is to be able to know what was the delta of acctinputoctets and acctoutputoctets that was received in this event , in comparison to the data that was already in the DB from the previous notifications. Can I somehow enter this delta into a different table that will show me the usage for each notification? Thanks Liran