After she’d been sick with flu-like symptoms for several weeks, Kirsti Hoopingarner decided to see her doctor. She’d been having trouble breathing, so her doctor ordered a chest X-ray and a CT scan.

The results showed a mass on Kirsti’s thymus gland. The 29-year-old was shocked. Her mother had been diagnosed with thymoma when she was 46. 

Kirsti had a biopsy of the mass done near her home in Illinois. While her local...