What Causes A Circular Corn At The Bottom Of The Foot?

I am 63 years old, weigh 140 lbs and have always gotten callus and corns. I have had a few of them removed. But for the last 6 years I have developed a circular corn on the bottom of my left foot. I went to a doctor and he put some black medicine on and told me that he couldn't do anything about it he told me the name of this kind of callus but I don't remember what he called it.