Early last year, at his home in San Juan, Puerto Rico, Jesus Apolinaris Cruz’s leg hurt so much he could barely sleep. “All day,” the 13-year-old recalls. “It was constant pain.” His parents took him to two local pediatricians, who examined him, drew blood, tested his platelets. No diagnosis. Finally, in April 2016, a physician ordered an MRI. No wonder Jesus’s leg hurt. He had a large, cancerous tumor lodged in his hip. …