It all started with just a tiny lump on his wrist.

In the summer of 2014, Kobee Cohen, then 8 years old, noticed a tiny growth on the underside of his right wrist. His grandparents, Hadley and Melinda Cohen, weren’t too concerned.

“What young, active boy doesn’t have a bunch of bumps and bruises?” Hadley recalls.

But they started to get a little nervous in January 2015, when Kobee banged his arm on a basketball goal...