The PSA level is often a good indicator of whether or not initial treatment was successful. Generally speaking, your PSA level should get very low after treatment. But PSA results aren't always cut and dry, and sometimes doctors aren't sure what they mean.
After surgery
The PSA should fall to an undetectable level within a couple of months after radical prostatectomy. Because some PSA may remain in the blood for several weeks after surgery, even if all of the prostate cells were removed, doctors often advise waiting at least 6 to 8 weeks after surgery before getting the test.
Blood tests have become much more sensitive in recent years -- so sensitive that they can detect very small amounts of PSA. This would seem to be a good thing, but it has made it more difficult to define exactly what an "undetectable" PSA level is. For example, a PSA of 0.5 (ng/mL) after surgery might be concerning, but doctors aren't sure whether this is also true of levels of 0.01 or 0.02. Some doctors would advise following such PSA levels over time to get a better idea of what may be going on, possibly with repeat tests every few months. Others might be more inclined to recommend further treatment. Of course, this uncertainty can be very stressful for patients and their families.
After radiation therapy
The different types of radiation therapy don't kill all of the cells in the prostate gland, so they're not expected to cause the PSA to drop to an undetectable level. The remaining normal prostate cells will continue to make some PSA.
The pattern of the drop in PSA is also different than with surgery. PSA levels after radiation tend to drop gradually, and may not reach their lowest level until 2 years or more after treatment.
Doctors tend to follow the PSA levels every few months to look for trends. A one-time, small rise in PSA might be a cause for closer monitoring, but it may not necessarily mean that the cancer has returned, as PSA levels may fluctuate slightly from time to time. However, a PSA that is rising on consecutive tests after treatment might indicate that cancer is still present. Some medical groups have proposed that a PSA rise of more than 2 above the lowest level it reached should be used as the cutoff point, but it's not clear if all doctors agree with this.
There is also a phenomenon called a "PSA bounce" that sometimes happens after brachytherapy. The PSA rises slightly for a short time within the first couple of years after treatment, but then falls back down. Doctors aren't sure why this happens, but it doesn't seem to have an effect on a patient's prognosis.
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