Mangy spot on dogs cheek

There is a raw spot on my dog's left cheek, about 1 inch in diameter, which
is not healing and if anything is slowly getting bigger. But I can't say
it's changing size much if at all. It looks just like a scrape or abrasion,
as if she had been rubbing that spot too much or somehow scraped her face.
Her fur is still there on the area, but thinner. It is a mottled red color.
Neither vitamin E nor cortisone cream have any effect.

I will take her to the vet but I thought one of you may be able to make a
diagnosis on the basis of the information I've given. Ideas?

Thanks,

Rich