Scientists from Queen Mary University of London have discovered that foot callouses/keratoderma (thickened skin) can be linked to cancer of the esophagus (gullet), a disease which affects more than 8000 people in the UK each year.
https://www.sciencedaily.com/releases/2017/02/170201093451.htm
Why thick skin develops on our palms and soles, and its links to cancer