Prostate-specific antigen (PSA) is a protein produced by both normal and cancerous cells within the prostate gland. A blood test measures its level. While PSA testing is commonly used for screening and diagnosis, its role shifts to monitoring treatment effectiveness and detecting potential recurrence after prostate cancer therapy. Understanding how PSA levels behave after radiation therapy is a key aspect of long-term prostate cancer management.
The Expected PSA Trajectory After Radiation
Following radiation therapy for prostate cancer, PSA levels typically do not drop to an undetectable range, unlike after surgical removal of the prostate. This is because radiation therapy does not remove the prostate gland; it targets and damages cancer cells, but healthy prostate cells remain and continue to produce some PSA. The decline in PSA after radiation is usually a gradual process, often taking months or even a few years to reach its lowest point.
The lowest point PSA reaches after treatment is called the “nadir,” and the time it takes to reach this can vary, often taking up to two years or more. If hormone therapy is administered alongside radiation, the PSA level may decrease more rapidly and reach very low levels within approximately six months.
Interpreting “Normal” PSA Levels Post-Radiation
Defining a “normal” PSA level after radiation therapy focuses on observing a stable, low trend, rather than a specific number. The objective is for PSA levels to fall and remain at a low level, indicating successful treatment. While a PSA nadir of 0.5 ng/mL or less is often considered a favorable outcome, a nadir of up to 1.0 ng/mL can also indicate successful control, especially in patients who did not receive hormone therapy.
A significant concern post-treatment is biochemical recurrence (BCR), meaning the cancer may have returned or was not fully eradicated. For patients treated with radiation, the most widely accepted definition of BCR is the Phoenix definition. This criterion defines recurrence as a PSA rise of 2 ng/mL or more above the lowest level (nadir) achieved after radiation. This rise needs to be sustained, typically confirmed by consecutive tests, to indicate a potential recurrence.
Common Fluctuations and What They Mean
Temporary PSA increases after radiation therapy are common and do not always indicate cancer recurrence. One such phenomenon is a “PSA bounce,” where the PSA level temporarily rises and then falls back down. This bounce typically occurs within the first few years after treatment, often around 12 to 18 months, and is thought to be related to inflammation in the prostate caused by the radiation. A PSA bounce does not signify that the cancer has returned or that treatment was unsuccessful.
Other non-cancerous factors can also lead to temporary PSA elevations. These include inflammation (prostatitis), urinary tract infections (UTIs), or benign prostatic hyperplasia (BPH). Certain medical procedures, like a recent prostate examination or catheter insertion, can also cause a transient PSA rise. Ejaculation can also temporarily affect PSA levels.
Ongoing Monitoring and Next Steps
Consistent PSA monitoring is standard follow-up care after radiation therapy for prostate cancer. Typically, PSA tests are performed every three to six months for the first five years after treatment, and then annually thereafter. This regular testing allows providers to track PSA trends and identify concerning changes.
If PSA levels show a consistent upward trend, especially meeting the Phoenix definition of biochemical recurrence, further evaluation is recommended. This may involve repeating PSA tests to confirm the trend, followed by additional diagnostics. These include advanced imaging like PSMA PET/CT scans or multiparametric MRI to pinpoint recurrent disease, or a prostate biopsy. Based on these findings, a doctor will discuss next steps, ranging from continued monitoring to salvage therapies like additional radiation, surgery, cryotherapy, or hormone therapy.