Cancer physicians for more than a century have followed the simple dictum that more is better—more surgery, more radiation, more chemotherapy and, most recently, more immunotherapy. But how much is enough? Do we escalate doses to the point of lethality, as those engaged in bone-marrow transplantation are forced to do regularly? Is this struggle to eliminate every patient’s cancer achievable or even warranted?

These questions have taken on a new urgency because oncology has lost sight of a basic principle: Every patient is...