Some experts in skin cancer regard Keratoacanthoma (KA) as a subtype of SCC, but with the difference that they may eventually resolve without treatment if left alone.
KA tumours appear as a rapidly growing scaley lump that appears on the arms and legs of elderly people, and can sometimes grow to be 2-3cm across in a matter of weeks. If left untreated, the lesion will usually stabilise in size after a period of 2-3 months, and then form a scaley plug in the centre, which eventually falls out and the lump starts to shrink and eventually disappear. However, they can be very difficult to distinguish from SCC, and if left untreated, the patient (and treating doctor) may eventually find out that it wasn't a KA and regret not cutting it out earlier, when the lesion was smaller and more easily removed. For this reason they are usually treated in the same way as a SCC with prompt surgical excision.