Former President Joe Biden has been diagnosed with an "aggressive" form of prostate cancer that has spread to his bones. But what does it mean for this type of cancer to be called aggressive?

“Last week, President Joe Biden was seen for a new finding of a prostate nodule after experiencing increasing urinary symptoms,” his personal office said in a statement. “On Friday he was diagnosed with prostate cancer, characterized by a Gleason score of 9 (Grade Group 5) with metastasis to the bone.”