how to delete old backup information?

Vista Home Premium SP1.
Using Backup and Restore to DVD-RW
I create a full (file) backup and then incremental ones until I have a dozen or so DVDs then I create another full backup.
I keep two such sets, overwriting DVDs as needed.
Question comes as how to delete the information about what is on the sets that no longer exist. If I do an advanced restore and ask for restore from older backup on this computer (with show more than 30 days checked), I get a very long list including those DVD's that no longer exist. There must be some way to remove that information but it is not obvious to me!

Any pointers appreciated.