Thomas Walsh beat cancer, took gold medal skier Mikaela Shiffrin to the prom and is eyeing Paralympics

The diagnosis arrived the day before Thomas Walsh left for a ski academy in Vermont.

For months, the rising star on the U.S. junior skiing circuit dismissed the ache in his right hip. After all, a snow-covered future beckoned. Walsh, then 14, planned to follow the example of Mikaela Shiffrin, his...