This year, nearly 1.7 million Americans will be diagnosed with cancer. Most will find out in the usual way; after having tiny blobs of tissue slurped up through a needle, smeared and stained on a slide, and put under the discerning eye of a pathologist. But starting this week, Medicare patients with advanced cancers will have access to a more 21st century diagnostic: Their cells can now be sequenced, matching patients with the drugs most likely to make a difference.