The term "malignancy" refers to cancerous cells that have the ability to spread to other sites in the body (metastasize) or to invade and destroy tissues. Malignant cells tend to have fast, uncontrolled growth due to changes in their genetic makeup.
Malignant cells that are resistant to treatment may return after all detectable traces of them have been removed or destroyed.
Moscow JA, Cowan KH. Biology of Cancer. In: Goldman L,Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier;2011:chap 185.