Kidney cancers can be cystic, and differentiating between a single benign cyst of the kidney and a kidney cancer can occasionally be a problem. When many cysts are present, as in polycystic renal disease, the distinction is easier. Cysts may also be caused by some parasitic infections.
It will actually collect the oxygen in the ambient air (~21%) and convert it into a concentration of around 85-94%. One of the main reasons you would want a concentrator over a tank and regulator would be when you are using a lot of oxygen for taking ozone saunas or ozonating oils.
If you wanted to calculate how much air is consumed by a person to get their needed 500-600, it gets very tricky, as only about 25% of the available oxygen is absorbed in an average breath, and there are a large number of confounding variables involved (depth of breath, exercise, age, health, etc).