Dandelion has been used medicinally for centuries for various things (Appalachian folk remedy is how I've usually heard it referenced). Nvere heard of it as a cancer treatment before though.

Some of those folk remedies become the basis for medicines -- think willow bark extract being further refined to be aspirin.

However --

remember laetrile? apricot seed extract, I think. Despite all the buzz, it seemed not to work.