Lois,
Long story short, necrosis occurs when a growing tissue exceeds its blood vascular supply. The center of the growing tissue has no energy supply so it dies, the cells die releasing their toxic genetics to be taken up by other cells.
This is not a good situation. Necrosis in a tumor means that the cells that have died due to lack of oxygen or blood supply have released their DNA to be taken up by other cells.

Not to hijack Oscar's thread as you suggested, e-mail me on this. I do have some info for you.

Cheers.

Mike