We think of cancer as if it were a singular entity, made up of identical cells. Unfortunately, the secret of malignancy of tumours and their resistance chemotherapy is that they consist of cells with quite different genomes. In oncogenetics, we speak of these as “clones”.
It turns out that the vast majority of cancers are composed of several different clones, one of which divides faster, another is more resistant to chemotherapy, and the third more easily regenerates after radiation therapy, for example.
Melanoma (a type of cancer that develops from cells called melanocytes) is the absolute record holder in this matter. A small black lump on our skin may consist of over 30 (!) different cell populations, which explains their astounding malignancy and ability to be reborn like a Phoenix from ashes even several years after the first line of treatment.