Prostate cancer is not just one disease; in reality there are several different types of prostate cancer, which will look different under a microscope and behave differently in your body. In addition to the more common types of prostate cancer there are also several kinds of rare cancers that can develop in the prostate.
Prostate cancer may be diagnosed from a biopsy slide or from a transurethral resection of the prostate (TURP) to treat an enlarged prostate. Often the rare prostate cancers are not given a Gleason Grade because they can behave differently from typical prostate cancer and can’t be measured in the same way.
In many instances the commonly used diagnostic tests may not be good at diagnosing these rarer types of prostate cancers, thus some of these rare cancers may not be diagnosed until after they have spread outside of the prostate and are causing symptoms. For instance, small cell prostate cancer doesn’t cause an increase in the PSA score so they often are missed by a PSA test.
Additionally, many of the rare cancers may be more aggressive than typical prostate cancer. Compounded by the fact that they might not be diagnosed by a PSA test and the fact they can more easily spread outside the gland they tend to be significantly more lethal.
It is also possible that you actually have more than one type of prostate cancer at the same time; this complicates both diagnosis and treatment.