It depends on what RTW ticket you have and if the ticket is partly used or not. I believe if the first segment has not yet been flown, you're eligble for a full refund minus some sort of service charge. Sometimes this could be the first continent.
If the ticket has begun already, I think it's much more difficult for a refund. I think they'll calculate each segment you've flown as a one-way ticket, add them all up; then give you back the difference. Of course, it depends on the type of RTW ticket.