context: When you have prostate cancer, your treatment plan and outlook depend on how far the cancer has spread at the time you're diagnosed. Doctors use a system called staging to figure out how the cancer affects your prostate and how far it has spread. Here's a look at how the system works and what the different stages mean. Prostate cancer grows within the prostate gland, often for many years. Over time, the cancer spreads outside the prostate. This happens in one of three ways: It grows into nearby tissues (invasion) It spreads through your system of lymph nodes and lymph vessels It travels to distant tissues through your blood (metastasis) Prostate cancer stages describe how far the cancer has spread. After a prostate cancer diagnosis, your doctor will do tests to see how far the cancer has spread. Not all men need every test. It depends on the results of your biopsy, a test that checks tissue from your prostate gland for cancer. Tests that help your doctor figure out the stage of your prostate cancer include: Digital rectal exam PSA (prostate-specific antigen) test Transrectal ultrasound MRI of the prostate CT scan of the abdomen and pelvis to see if the cancer has spread Nuclear medicine bone scan to see if the cancer has spread to your bones Surgery to examine the lymph nodes in your pelvis for prostate cancer spread Cancer cells don't look the same as healthy cells. The more different they appear, the more aggressive the cancer tends to be. The Gleason system uses the numbers 1 to 5 to grade the most common (primary) and second most common (secondary) patterns of cells found in a tissue sample. Grade 1: The cells look very much like normal prostate cells. Grades 2-4: Cells that score lower look closest to normal and represent a less aggressive cancer. Those that score higher look the furthest from normal and will probably grow faster. Grade 5: Most cells look very different from normal. Doctors add your primary and secondary numbers together to form your total Gleason score. That tells you how aggressive the cancer is. The lowest score for a cancer is 6, which is a low-grade cancer. A Gleason score of 7 is a medium-grade cancer, and a score of 8, 9, or 10 is a high-grade cancer. Generally, the higher your Gleason score, the more aggressive the cancer. That means it's more likely to grow and spread to other parts of your body. Doctors use this information, along with the stage of the cancer, to choose the best treatment for you. As they do for most cancers, doctors use the TNM system to describe prostate cancer stages. The system uses three different aspects of tumor growth and spread: T = tumor: What's the size of the main area of prostate cancer? N = nodes: Has it spread to any lymph nodes? If so, how far? M = metastasis: How far has the prostate cancer spread? Doctors combine the T, N, and M results with the Gleason score and PSA level in a process called stage grouping. The system uses Roman numerals from I (the least advanced) to IV (the furthest along). The stage helps your doctor choose the best course of treatment for you. Stage I The cancer is growing in your prostate, but hasn't yet spread beyond it. In most cases, the doctor can't feel the tumor during a digital rectal exam ( DRE) or see it in imaging tests. The Gleason score is 6 or less and the PSA level is less than 10. The tumor is in one-half or less of only one side of the prostate. Stage IIA The cancer is growing in your prostate but hasn't spread beyond it. The doctor may or may not be able to feel the tumor during a DRE or see it on an imaging test. The tumor can touch more than half of one lobe of the prostate, but doesn't involve both lobes. The Gleason score is 7 or less and the PSA level is less than 20. Stage IIB The cancer is growing in your prostate but hasn't spread beyond it. The doctor may or may not be able to feel the tumor during a DRE or see it on an imaging test. The tumor can be in one or both lobes of the prostate. The Gleason score is 7 and the PSA level is less than 20. Stage IIC The cancer hasn't spread beyond the prostate. The doctor may or may not be able to feel the tumor during a DRE or see it on an imaging test. The tumor can be in one or both lobes of the prostate. The Gleason score is 7 or 8 and the PSA level is less than 20. The cancer cells appear more abnormal than in stage IIB Stage IIIA The cancer hasn't spread outside the prostate. The doctor may or may not be able to feel the tumor during a DRE or see it on an imaging test. The cancer hasn't spread to any lymph nodes. The Gleason score is 8 or less and the PSA level is at least 20. Stage IIIB The cancer has spread outside the prostate but hasn't made it to the lymph nodes or distant sites in the body. The Gleason score is 8 or less and the PSA is any level. Stage IIIC The cancer may or may not have spread outside the prostate. The cancer hasn't spread to any lymph nodes. The Gleason score is 9 or 10 and the PSA is any level. Stage IVA The cancer may or may not have spread to tissues near the prostate. The cancer has spread to nearby lymph nodes but not to distant sites in the body. The Gleason score and PSA are any value. Stage IVB The cancer may or may not have spread to tissues or lymph nodes near the prostate. The cancer has spread to distant sites in the body such as lymph nodes, bones, or other organs. The Gleason score and PSA are any value. It's important to get the stage right because it plays a huge role in choosing the best treatment for you, as well as your outlook. For this reason, it's worth going through extensive testing to get the correct prostate cancer stage.