Keratoacanthoma. Image 1 of 2. Large keratoacanthoma, also known as molluscum sebaceum, on the back of a patient's hand. Keratoacanthomas are harmless skin nodules that most commonly develop on the face and arms of elderly people. They grow rapidly and in eight weeks can reach a diameter of 2 centimetres. They slowly disappear without treatment, but some patients prefer to have them surgically removed. Their cause is unknown, but may be related to sunlight exposure. Physically they are similar to skin cancers, and a histopathological study (study of diseased tissue) may be required to distinguish one from the other. See M190/088 for the same hand seen from above.