Sonia Keuroghlian’s husband, a radiologist, has seen cancer in people in their 20s and 30s, younger than the ages recommended for most publicly funded cancer screening. So, when the Toronto couple turned 40, he suggested they undergo full-body MRI (magnetic resonance imaging) scans at a private clinic in Vancouver, where her husband is from, and where they were vacationing in August 2018, “just to make sure everything is OK,” Keuroghlian said. She was healthy, active, fit, and feeling perfectly normal, which is why, when the radiologist later pulled up an image of a 10.5-centimetre...