The appearance and arrangement of cancer cells allow physicians to determine the grade of your cancer.
The American Cancer Society - Prostate Cancer Stages and Grades
In order to fully understand your prostate cancer, your healthcare team will need to determine the grade of your cancer. Grading involves looking at a tissue sample from the prostate under a microscope to view the tumour cells. The appearance of the tumour cells allows your doctor to see if you have a low grade or a high grade cancer, which will help in the decision between the different treatment approaches.
The grade of your cancer indicates how similar or different the cancerous cells are with respect to healthy prostate cells. Cells that are “well differentiated” are most similar to healthy cells whereas cells that are “poorly differentiated” look very different. These poorly differentiated cells are more likely to behave aggressively.
The microscopic structures of the prostate are also important. Normal prostate tissue consists of a structured arrangement of small glands (acini) and ducts. In low grade cancers, the structure resembles the normal prostate, whereas in high grade cancers the structure is almost indistinguishable and the arrangement of the cells do not form the normal structured pattern and appear haphazardly arranged.