[thoxahr]

Blood contains immune system elements which would likely reject cancer samples from a different donor. Having to obtain blood and cancer sample from the same donor would complicate things quite a bit.

In addition, it doesn't seem as though they are able to recycle the medium. Frequently draining and refilling with fresh blood seems rather wasteful.

[rleigh]

The immune cells are trivial to remove; a few minutes in a centrifuge is all that is required. Even antibodies can be separated out pretty easily (though I haven't done it myself). The problem is the quantity and quality of the product. And blood alone isn't sufficient; it doesn't contain enough glucose and other metabolites to sustain cells for long (since it's continuously circulating and levels are maintained by the body, which isn't true for a culture flask).