My 53 Year Old Brother Has Recently Been Diagnosed With

My 53 year old brother has recently been diagnosed with a Stage II or III inoperable astrocytoma in his left temporal lobe. It is too diffuse and in a critical location to be resected. He will undergo radiation therapy soon. I also know a man who has undergone a hemispherectomy who is fully functional: he is an accountant, drives a car, and has a family. I'm wondering how humans can undergo a hemispherectomy and not only survive but thrive and yet some glioma patients cannot have their tumors removed. It seems the hemispherectomy would have caused many more deficits than a tumor resection and yet it did not. Can you explain? Thank you.