1 The degree of difference between the cancer cells and normal cells. 2  How different your cancer cells look from normal cells is called your cancer's grade. 3  Breast cancers are graded on a 1 to 3 scale, with grade 3 cancers being the most different looking and considered the most aggressive. How different your cancer cells look from normal cells is called your cancer's grade. 2  Breast cancers are graded on a 1 to 3 scale, with grade 3 cancers being the most different looking and considered the most aggressive.