Former president Jimmy Carter announced Sunday that he’s stopping treatment for his brain cancer at the ripe age of 91. Inspiring as his story is, it’s hard not to wonder: how’d he do it? Turns out Carter was being treated with Keytruda, a new and highly promising immunotherapy drug that was approved not long after Carter was diagnosed. In August 2015, Carter announced that his aggressive melanoma — a deadly form of skin cancer — had spread to his brain, and it seemed pretty clear that he had only a few weeks left to live. Still, he underwent treatment at...