That's why the good Lord gave (most of) us two kidneys.
Hopefully the tumor is removable, non-metastized, and they can salvage the kidney. At least you have an healthy spare.
My next door neighbor found out she had a fatty tumor totally consumed one of her kidney's when she was pregnant. They managed to successfully remove it, and 4 months later she delivered a healthy baby girl. But now she's only got one kidney and they don't recommend she have any more children.