I believe it has a lot to do with the location of the tumor. I think that only in certain, special cases, can they just remove the mass itself. Again, probably due to the location of it, and also removing just the tumor in many cases would also involve cutting through many blood-vessels (that alone making it very complicated). And most importantly, they remove the entire kidney to prevent any ''potential'' malignant cells from spreading to other areas of the body. Anyway, like I said, I'm definitely NOT an expert on this and I'm basically repeating what my husband's urologist told him. Each case is different and only your doctor can decide the proper course of treatment, after completing necessary diagnostic tests. Sometimes malignancies can only be definitely confirmed once the kidney ''is'' removed, as was my husband's case. Sometimes a biopsy is performed, but again, I'm not sure under what circumstances this is done.
There are lots of well informed people here, and they can probably give you better information than me. Do question the doctor. Make a list of questions to ask, at the next appointment.
Keep us posted regarding what happens.