Prostate-specific antigen (PSA) is a protein produced by prostate cells. Measuring PSA levels in the blood is a standard method for monitoring prostate cancer, particularly after radiation therapy. Understanding how PSA levels behave helps assess treatment effectiveness and identify potential concerns.
Expected PSA Response After Radiation
Following radiation therapy, PSA levels generally decline gradually, unlike the immediate drop seen after surgical removal of the prostate. This slower decline occurs because radiation kills cancer cells over weeks or months, and healthy prostate tissue remains, producing small amounts of PSA. The lowest point PSA reaches after treatment is called the “nadir.” A common goal is a PSA nadir of less than 0.5 ng/mL, though a low but measurable PSA level is normal.
During this decline, a temporary rise in PSA, known as a “PSA bounce,” can occur. This bounce is not indicative of cancer recurrence and is observed in about 31% of patients. It typically happens within the first few years after treatment, often around 16 to 18 months post-radiation, with PSA usually returning to its declining trend or stabilizing. The bounce amplitude is generally around 1.3 ng/mL. While its exact cause is not fully understood, it is thought to relate to inflammation from radiation.
Defining Biochemical Recurrence
A concern after radiation therapy is biochemical recurrence, a rise in PSA levels suggesting cancer may have returned. The “Phoenix definition” is the most widely accepted standard for this, stating that biochemical failure is a PSA rise of 2 ng/mL or more above the lowest PSA level achieved (nadir).
This definition provides a consistent standard for evaluating treatment outcomes. A single rising PSA value does not automatically confirm recurrence. Healthcare providers look for a consistent trend of increasing PSA levels. Trends in PSA values over time are more informative than isolated readings, and the date of failure is considered “at call” rather than backdated to avoid misleading interpretations of treatment success rates.
Other Factors Affecting PSA Levels
While a rising PSA can signal cancer recurrence, other non-cancer-related factors can also cause temporary fluctuations. Inflammation within the prostate, such as delayed radiation prostatitis, can lead to transient PSA increases. This inflammation is a known side effect of radiation and can occur months or years after treatment, causing PSA to temporarily rise before declining again.
Infections, such as urinary tract infections, can also cause PSA levels to rise. These infections irritate the prostate, leading to a temporary increase in PSA production. Certain medical procedures, like a prostate biopsy, can similarly elevate PSA levels for a short period. Therefore, a single elevated PSA reading should be interpreted cautiously, and doctors often consider these other possibilities before concluding cancer has returned.
Monitoring and Long-Term Follow-Up
Regular PSA monitoring is standard practice following radiation therapy to track effectiveness and detect recurrence. PSA levels are typically checked every 3 to 6 months for the first two years. If levels remain stable and low, testing frequency may decrease to once a year.
Long-term follow-up is important as PSA recurrence can occur many years after treatment, sometimes even up to 20 years later. Patients should discuss PSA trends with their providers, as the rate and consistency of changes provide valuable information. This ensures that any concerning trends are addressed promptly, potentially leading to further diagnostic tests or adjustments.