Untreated squamous cell carcinoma of the skin can destroy nearby healthy tissue, spread to the lymph nodes or other organs, and may be fatal, although this is uncommon.

Is squamous cell carcinoma the most deadly?

From the Harvard Health Letter, May 2006

It's three times as common as melanoma (some 200,000 new cases each year versus 62,000). Though not as common as basal cell (about one million new cases a year), squamous cell is more serious because it is likely to spread (metastasize).

Can squamous turn into melanoma?

Squamous cell skin cancer can be quite serious in a minority of cases, but it does not “turn into” melanoma. Melanoma is a deadly cancer that arises from melanocytes, a different type of skin cell than squamous cells.