How often does a brain tumor save your life? In Kellilyn Monar’s case, it may have done just that. She’d planned to be at the Route 91 Music Harvest Festival in her hometown of Las Vegas on Oct. 1, 2017. But a brain tumor diagnosis interrupted those plans.

“I go every year, but because my surgery was scheduled that week, I called to get a refund on my tickets,” she says. When the concert became the site of the deadliest mass shooting...