Arantza: It is muy importante that your sister is treated by cancer specialists, and I see that M.D. Anderson has a cancer center in Madrid. Anderson is one of the top 5 U.S. cancer centers.
Most doctors will treat cancer, but few doctors specialize only in cancer. Oral cancer represents a small percent of cancers so a doctor with experience in "cancer" most likely will not be an expert in all the little differences in oral cancer.

We wish you and your hermana the best of luck. TLC