MR/MRS/MS/DR are all optional on airline tickets (I think the only purpose is to identify the gender of the passenger so that appropriate amenity kits could be loaded in F/J?).

However, MSTR/MISS are sometimes used to identify that a ticket is a child fare (so that an adult won't be able to use it).

If you are not a medical doctor, you should not have DR on your ticke as they flight attendants may come to you if there is a medical emergancy onboard.