Stephen De La Torre, 7, is usually all smiles. But on a recent Wednesday at Stanford Medicine’s cancer center in Palo Alto, the boy climbed resolutely into the chair of a first-of-its-kind treatment device, focused for the task ahead. De La Torre had been diagnosed with an unusual form of cancer, a rare tumor attached to his brain stem. His hair, just an inch long, had begun to grow back after emergency surgeries and a course of radiation. Scars were still visible on his scalp.