Herceptin & expriation date?

My mother has weekly Herceptin infusions for almost a year now. They only weigh her on the days she sees the onc and never change her dosage due to a change in weight. She's lost about 10-15 pounds in the last year, and her dosage has never changed. DOesn't have anything to do with the expiration date, but I thought you might find it interesting that some centers don't mix the bag based on weight.