I too have had kidney masses (4 of them), and
they were caused by Autoimmune Pancreatitis
. I had a biopsy to confirm it, and
after treatment they have disappeared. I was also really scared that they were cancer, because I had a huge mass on my pancreas as well. You said you had GERD like symptoms, and that makes me curious. Have you had an abdominal CT yet? I was experiencing pain in my left side, especially after eating. Sometimes the pain went into my back.