Grade, how do they work it out?

Hi, my diagnosis was stage 1 BC no spread into lymph nodes & I had a lumpectomy & on anastrozole for 5 yrs, plus 3 wks radiotherapy coming up soon. Just wondering how they work out the grade of tumour, mine was grade 2, intermediate. The grades are 1 slow growing, 2 intermediate & 3 fast growing. Mine was 10mm in size & found at routine mammogram so presume either wasnt there at previous screening 3 yrs ago or was there & too small to show up. Just wondering how they decide what grade they are, us it based on fact it didnt show up before so they calculate the growth rate based on that, or is it when they look at the pathology of the tumour?

On the pathology of the tumour I understand. It is in relation to the difference to normal breast cells and how fast growing they are. My diagnosis was the same as yours but I had further surgery as the margins were not clear.