Prostate specific antigen (PSA) is a protein made by cells in the prostate gland. The level of PSA in the blood is usually, but not always, increased above normal levels when there is a cancer in the prostate gland.

If the prostate gland, and the cancer within it, is completely removed by an operation (a radical prostatectomy) then the PSA level should go back to very low levels afterwards.

Prostate cancers can spread outside the prostate gland, either into the surrounding tissues or by sending seedlings in the blood stream to other parts of the body forming secondary cancers (usually when this happens the secondary cancers develop in one or more of the bones but occasionally other places can be affected as well).

In its early stages, when the spread into the nearby tissues or the secondary cancers are microscopically small, there will be no obvious signs of tumour. But the cancer cells will produce PSA and the level of PSA in the blood will start to rise.

So checking the PSA regularly after surgery (or radiotherapy) for an early prostate cancer is a way of detecting early signs of either a recurrence of the growth in the tissues around where the prostate was or spread of the cancer to other places like the bones.