Scary to read for me in particular, as my little cousin just passed away from this. Luckily, that is fairly unusual now, but still terrifying for him and his family. This is probably pretty bad for an athlete to get and continue, but I am not a doctor and really dont know. Hope he gets well soon, he is on the older side to have this diagnosed, I will let medical experts on the board comment on that effect.
Good they found it.
So, off topic, but well, a personal issue. If you havent please consider registering to donate blood marrow. I have matched 3 times in 20 years, and the rumors you hear are correct, it does freaking hurt. That said, it si a very small price to pay to help someone. Takes 20 minutes to register, your doctor can help you, they do have to draw blood to do it as I remember.