I've read more then a few independent case studies where marijuana extracts where used to treat cancer as a last resort and had amazing results. However, the extracts were applied directly to the cancerous area, not smoked.

I think in the last one I read, involving brain cancer, researchers believed they had discovered exactly how the THC caused the cancer cells to die off: they become so "hungry" they "ate" their own internal cell structures, causing them to slowly shrink to death over a period of months.