When I found out I had thymoma — a rare tumor of the thymus gland — I knew I couldn’t stay in South Texas for my treatment.

I’d been convinced for more than a decade that something was wrong with me, but nobody ever connected any of my symptoms with the mass growing slowly behind my sternum.

Over the years, I’d seen a cardiologist for heart palpitations, a gastroenterologist for swallowing problems, an ophthalmologist for...