This is a common treatment for actinic keratoses utilizing liquid nitrogen (the same chemical that makes dry ice) in a freezing cold spray. It works with the freeze-thaw cycle: the spray freezes the abnormal cells; then, as they thaw, the cells expand and explode. This can effectively spot treat small lesions. Cryotherapy is very painful and can leave white scars where the pigment has been blasted by liquid nitrogen. For this reason, doctors don't use it for removing precancers as much as I do a TCA peel spot treatment.
Find out more about this book:Simple Skin Beauty: Every Woman's Guide to a Lifetime of Healthy, Gorgeous Skin