No one knows about my cancer, but I can relate to your question. Its the same thing as when people ask me when I'm getting married, or do I want to meet their cousin ("She's divorced and just CAN"T find anyone!" (Wonder why! And who do you think I am--any port in a storm?)).
Say thank you and take it for the good intention with which it is meant, even if you don't feel that is an honest response. They mean well for you. Allow yourself to be thankful they care enough to try and make you feel better.