Hi Penelope;
It is a measure of the percentage of cells undergoing mitosis. They determine this by looking at the cells under high powered viewing field of a microscope. 2/10hpf means that 2 out of 10 cells viewed under the high powered lens (400x) showed active cell division. eg., 20% of your tumor cells were dividing which would give you a mitotic index of 1 on the nottingham grading scale--eg. which is good not very many of the cells were dividing.
you might fine this link useful in regard to how they grade your tumor
http://ccm.ucdavis.edu/bcancercd/311...g_diagram.html
kk1