They actually are immortal as long as they have live cells to attach to. Scientists have a cancerous tumour that has been alive for over 50 years. The reason why they can live for a very, very long time is because they are a mutation and they don't get a signal to die so they keep so they will keep multiplying until there's nothing left. The mutation takes place in our genes.
Cancer cells behave just like cells in an embryo. They grow quickly but they never get they protein signal to the genes to turn off as normal cells would. If we new how to manipulate those chemical signals to specific genes we could "turn off" cancers. They aren't immortal. Hard to isolate and quite varied.
Not exactly immortal rather indestructible. The cancer cells get a renewed life every time they turn a regular cell into a cancer cell. The very function of the cancer cell is to corrupt the fresh cells until 100% of cells are turned into cancer cells. Unless detected in the very early stage u can't separate these cells from the regular cells.