The pathologist grades a tumor based on how the cells look under the microscope. The tumor cells can appear close to normal (low grade) or more aggressive and angry (high grade). As you might expect, low-grade tumors tend to have a better prognosis than high-grade tumors. Most pathologists grade tumor biopsies as I, II, and III instead of high and low.
The grade of the tumor allows us to predict who will go on to develop invasive tumors and who will not; 6% of grade I tumors, 52% of grade II tumors, and 82% of grade III tumors will become invasive. It is harder to define three different grades for a cytology specimen, and thus most urine cytologies are simply reported as high grade or low grade.