My dr thought rash was ring worm, not sure, very itchy and is spreading. i' m concerned of skin cancer, would skin cancer spread fast like that?

A biopsy would tell: No, not usually. I would recommend that you go with the treatment that your doctor gave to you first, and if that isn't helpful within a couple of weeks, you should ask for a biopsy. This can be done easily, and most family doctors do this. Then you'll know for sure if it is a skin cancer.

It's ringworm. : Actually a fungal infection not a worm. Treat with an anti fungal tablet and cream and will likely be gone within the week.

Unlikely: However, skin cancer can occasionally spread very quickly. If the lesion is not improving with proper therapy, a biopsy is definitely recommended.

Not usually: If you are concerned about the diagnosis, the safest thing to do would be to seek a second opinion. Sometimes having fresh eyes look at the problem can be a great thing.

Biopsy : If you are concerned of the possibility of a skin cancer, a small skin biopsy can be done and the results are usually ready within a week and can easily tell the difference between skin cancer and different rashes. Consult a dermatologist for evaluation and treatment.