These proteins normally secreted into the ejaculate leak into the blood because of the abnormal cell architecture found in the tumor. They can then be detected in the serum by sensitive immunoassay methods. So a man can have an elevated blood PSA or PAP and not have cancer. By definition, a cancer cell not only has the capacity for abnormal growth but also possesses the ability to invade surrounding tissue and metastasize to distant sites. Death from cancer is the result of a cancer cell taking root and growing in a critical site causing the cancer patient to become irreversibly debilitated and causing his eventual death.
Prostatic cancer can metastasize in a number of ways.
By growing in size it can push its way into surrounding tissues by direct extension and protrude beyond the confines of the capsule that surrounds the prostate or into adjacent organs such as the seminal vesicles and nearby lymph nodes. In order to grow in size a tumor needs to increase its blood supply. Prostate cancer like most cancers induces new blood vessel formation which provide nutrients to the growing tumor. Prostate cancer cells can also breakdown the membrane barriers to these blood vessels allowing the tumor to gain access to the bloodstream. In cancer of the prostate it is thought that bone metastasis are the result of hematogenous dissemination.