How to calculate the mitotic rate? - Should I calculate the area of a High Power Field (HPF)? (May/05/2010 )

I've counted the number of mitotic cells (H & E slides) in 10 HPF (40x), The AJCC cancer staging manual recommends that mitotic rate should be determined by the “hot spot” approach and expressed as the number of mitoses per square millimeter of primary tumor.
Should I calculate the area of the HPF so I can know the number of mitotic cells/mm2?

Thanks a lot!

You could do this, however it may be easier to take a picture and use a calibrated scale (stage micrometer).