Prostate cancer is easiest to treat while it is confined to the prostate. At this stage, surgery and radiation are most likely to be curative and completely kill or remove whatever cancer cells are present. If left untreated, however, prostate cancer can proceed on a number of different paths.
It is well known that, sometimes, even when untreated, prostate cancer simply does not continue to grow or only grows at an extremely slow rate. In fact, some studies have shown that prostate cancer is present in the majority of older men at the time of their deaths even if they had not been diagnosed. This suggests that many older men have prostate cancer that is small and slow-growing, but the disease causes them few symptoms and does not lead to their death. Sometimes, your doctor may actually advise you to put off treatment for now - either because your cancer is thought to be of the slower-growing type or because your general health is such that treatment is not a good idea. This approach is called "watchful waiting".
Many cases of diagnosed prostate cancer, however, if left untreated, will grow and possibly spread outside of the prostate. The first sites of spread are typically to the nearby tissues. The cancer can spread down the blood vessels, lymphatic channels, or nerves that enter and exit the prostate, or it can erode directly through the capsule that surrounds the prostate. The seminal vesicles are a site of particularly common early spread. More extensive “local” spread can occur with cancer invading the nearby bladder or rectum.
Further advancement of the cancer can occur when cancer cells enter the blood vessels and lymphatic channels and then spread to distant areas of the body. Once the cancer has entered into these vessels, prostate cancer cells can “seed” into virtually any other part of the body. Prostate cancer is known to have a particular affinity for spreading or metastasizing to bones, especially the lower spine, pelvis, and femur. Other organs, such as the liver, brain, or lungs, can also be the sites of spread, but these are more rare.
Once the cancer has spread distantly from the prostate, it becomes far harder to effectively treat and completely cure. Chemotherapy and hormone therapy become the most feasible options because they treat cancer throughout the body.
If left to spread without intervention, or if treatment has failed, then prostate cancer can ultimately result in death.