This is something I've never encountered before. Apparently in some parts of the world, notably in the UK, surgeons are referred to as "Mister" or "Ms." instead of "Doctor". Minnesotastan ran across a literary reference in which a surgeon was offended when called "Doctor." Commenter Phyllis says:

Yes. In the UK, surgeons used to be LESS THAN doctors (who were educated and gave out medicines and didn't get their hands dirty like those plebeian surgeons).

Even though they are more like the US now, where they are doctors PLUS extra training, it seems that the term has persisted.

Now I wonder about other countries. How do people refer to medical practitioners and surgeons where you come from? Link

(Image credit: Flickr user  Salim Fadhley)