Inquiry Reflection - What I have learnt about Osteosarcoma

I have learnt that only 1-3 people get it New Zealand a year. My brother has had Osteosarcoma.  His operation took 7 hours. I learnt what chemotherapy does to your body. My brother had his whole fibula bone (the bone between the knee and ankle) removed from his leg and he can still walk. Osteosarcoma mainly affects boys. The question I still have is: how did the tumor get to his leg in the first place?