Complete (radical) nephrectomy is surgery to remove all of a kidney. Kidney-sparing surgery (partial nephrectomy) removes part of a kidney.
White matter signal changes on the MRI essentially means that on the MRI, the white matter showed some scattered bright spots. White matter in the brain refers to the fiber tracts that carry information to and fro from the brain. My first question when somebody asks me what next and what does this mean is to ask them why was the MRI done in the first place.
In theoretical terms, further study of white matter and its disorders expands our knowledge of the brain as an extraordinarily complex structure in which the connectivity provided by white matter is central to cognition, emotion, and consciousness itself.