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Radiation Therapy · Scan & Report Explainers
PSA Nadir After Prostate Radiation — What Your Lowest Number Actually Means
Medically reviewed by Dr. Gangadhar Vajrala, Radiation Oncologist, MBBS · MD (Radiation Oncology) · MPH · Last reviewed August 2026
If you have had radiation for prostate cancer, one number now follows you from one follow-up appointment to the next: your PSA. Somewhere in your report or your consultation notes sits the word nadir — and often nobody stops to explain it. Nadir means lowest point. It is the smallest PSA value you have recorded since treatment ended, and it becomes the reference line your team measures every later reading against, including under the Phoenix definition. This page explains that vocabulary once, properly, so you are not decoding it alone every three months.
- Nadir just means your lowest point — It is the smallest PSA value recorded since radiation ended — identified looking backwards, not a target set for you in advance.
- PSA is not expected to reach zero — Your prostate is still in place after radiation, so normal tissue keeps making a small amount of PSA. A low detectable number is the expected result.
- The fall is slow, and that is normal — Reaching nadir commonly takes around 18 months to three years, sometimes longer — a gradual decline is not a sign that something has gone wrong.
- Phoenix is the rule your team applies — A rise of 2 ng/mL or more above your nadir is the agreed ASTRO threshold for further assessment — deliberately set high so a temporary bounce is not misread.
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The short answer
What does PSA nadir mean after radiation?
Nadir means lowest point. Your PSA nadir is the smallest PSA value recorded since your radiation finished, measured in nanograms per millilitre (ng/mL). It is not a target set for you in advance. It is a number your team identifies looking backwards, once your PSA has stopped falling and settled into a stable pattern.
PSA, or prostate-specific antigen, is a protein made by prostate cells — both healthy ones and cancerous ones. Radiation treats the prostate in place rather than removing it, so healthy gland tissue survives and carries on producing a small amount of PSA. That is why the nadir after radiation is normally a low but still detectable figure, and not zero.
Once that lowest point is reached, it stops being just a result and becomes a reference line. Every PSA test you have afterwards is read against it. That is the reason your nadir gets quoted back to you at appointment after appointment, sometimes for years.
This page explains the vocabulary that appears on prostate follow-up reports. It does not interpret any individual result — only your treating team, with your full history in front of them, can tell you what your own numbers mean.
Why the rules are different
Why does PSA behave differently after radiation than after surgery?
Most confusion about nadir comes from applying surgery expectations to a radiation result. The two follow completely different patterns, and they are judged by different rules.
| What you are comparing | After radiation therapy | After surgical removal of the prostate |
|---|---|---|
| The gland itself | Stays in place and is treated where it sits | Removed completely |
| Expected PSA over time | Falls gradually to a low, detectable nadir | Expected to become undetectable |
| How long the fall takes | Commonly around 18 months to three years, sometimes longer | Weeks |
| What a small number means | Expected — normal prostate tissue is still making PSA | Prompts a closer look, since none is expected |
| The rule used to flag a rise | The Phoenix definition: nadir plus 2 ng/mL | A separate threshold your urology team applies |
| Temporary rises | A PSA bounce is well recognised and often settles on its own | Not an expected pattern |
If you are coordinating a parent’s follow-up from another city or another country, this is the single most useful thing to understand before the next report lands in your inbox: a detectable PSA after radiation is not the same finding as a detectable PSA after surgery.
Did you know?
The Phoenix definition is named after the city, not a person. It was agreed at the 2005 RTOG-ASTRO consensus conference held in Phoenix, Arizona, and replaced an earlier 1996 rule that had defined failure as three consecutive PSA rises. The change was made partly because the older rule flagged temporary PSA bounces as failures. The Phoenix threshold, nadir plus 2 ng/mL, remains the standard reference in ASTRO and NCCN follow-up guidance as of 2026.
Reading the number
What is a good PSA nadir value after radiation?
There is no single universal cut-off. Many radiation oncologists describe a nadir under about 0.5 ng/mL as a reassuring result, and a lower nadir is generally viewed more favourably than a higher one. But the figure your own team treats as good depends on the context around it, not the number alone.
Three things change that context. Your risk group at diagnosis is one. The radiation technique used is another. And whether hormone therapy formed part of your plan is the third — hormone therapy suppresses PSA while it is active, so a very low reading during that period may settle at a different level once the therapy ends. A nadir recorded under hormone therapy is not read the same way as one recorded without it.
What experienced teams watch most closely is not a single figure but a pattern: is the number low, and is it stable across repeat tests? A steady 0.4 ng/mL across four visits tells your oncologist far more than any one reading on any one day.
Comparing your nadir with another patient’s is rarely useful. Two men with the same figure can be in very different clinical situations. Bring your own values, with their dates, to your treating team.
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The rule behind the number
What is the Phoenix definition?
The Phoenix definition is the rule most teams use to decide when a PSA rise after radiation counts as a biochemical recurrence. It sets the threshold at a rise of 2 ng/mL or more above your nadir — which is why you will often see it written simply as nadir plus 2.
A worked example makes it concrete. If your nadir settled at 0.4 ng/mL, the Phoenix threshold for you is 2.4 ng/mL. A reading of 0.9 does not meet it. A reading of 1.6 does not meet it. A reading of 2.5 does. Reaching the threshold triggers further assessment by your team — repeat testing and a wider review — and is not, by itself, a conclusion about your disease.
The threshold was set deliberately high. Under the older 1996 rule, three consecutive rises counted as failure, and that flagged a great many men who were experiencing nothing more than a temporary bounce. The 2005 RTOG-ASTRO consensus in Phoenix, Arizona replaced it with a single, higher, clearer line.
One detail often missed: under Phoenix, the date of recurrence is recorded as the date the threshold is actually crossed. It is not backdated to when the rise first began. If you are reading a report that mentions a date alongside a biochemical recurrence, that is what the date refers to.
Guideline bodies including ASTRO and NCCN continue to reference this threshold in prostate follow-up guidance as of 2026.
What happens over the years
How your PSA is tracked after radiation, step by step
The same broad sequence applies whatever your own numbers turn out to be — only the pace and the next step change.
A baseline is recorded before treatment
Your PSA before radiation begins is the figure everything afterwards is compared against, alongside the rest of your diagnostic workup.
PSA is retested at set intervals
Typically every three to six months in the early years, then less often. Your team sets your own schedule based on your risk group and your plan.
The number drifts down over months, not weeks
Treated cells die off gradually, so the decline is slow by design. A gradual fall is the expected pattern, not a warning sign.
The lowest reading becomes your nadir
Once the decline flattens out, the lowest value recorded is identified as your nadir and becomes your personal reference line.
Later readings are measured against nadir plus 2
Small movements above nadir are common and often settle. Crossing the Phoenix threshold prompts repeat testing and a closer look.
You hear the interpretation from your team
In conversation, with your full history and imaging in front of them — never from a single number on a lab printout alone.
Three words, often confused
Nadir, bounce and biochemical recurrence — what is the difference?
These three terms turn up in the same conversation and mean quite different things. Getting them apart is most of the battle.
Nadir
Your lowest recorded point
The smallest PSA value since radiation ended. A reference line, not a verdict. It is identified in hindsight, once your PSA has settled.
PSA bounce
A temporary rise that settles
A brief rise that falls back without any treatment. Recognised more often after brachytherapy and in younger patients, usually within the first couple of years.
Biochemical recurrence
Crossing the Phoenix threshold
A rise of 2 ng/mL or more above nadir. It prompts further assessment and repeat testing — it is a trigger for investigation, not a diagnosis on its own.
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Putting it together
Reading a sample PSA follow-up line, term by term
Here is how the wording on a typical prostate follow-up note breaks down. Bring your own report to your consult and a radiation oncologist will go through your exact phrasing with you.
| What you might see | What it means |
|---|---|
| PSA 0.4 ng/mL | Your current prostate-specific antigen level, in nanograms per millilitre — one point on a trend, not a standalone verdict |
| Nadir 0.3 ng/mL (Mar 2025) | The lowest value recorded so far, with the date it was measured. This is your personal reference line |
| Stable compared with prior | Your clinician has compared this reading against the last one rather than reading it in isolation |
| No evidence of biochemical recurrence | Your readings have not crossed the Phoenix threshold of nadir plus 2 ng/mL |
| Repeat PSA in 3 months | The trend is being followed at intervals — routine surveillance, not an alarm |
| Discussed with treating consultant | A note confirming your result has already been reviewed in the context of your full case |
If you are back at work and fitting appointments around a job, it helps to keep every PSA value and its date in one list on your phone. Your team can read a trend in seconds from a list like that — and you will spend far less of a short consultation reconstructing dates from memory.
You do not have to decode this alone
One conversation can make your PSA report make sense
Whether you have just seen your nadir for the first time or you have been tracking PSA for years, a radiation oncologist can explain your own numbers in plain language.
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Common questions
Your PSA nadir and follow-up report — questions answered
What does PSA nadir mean after radiation therapy?
Nadir simply means lowest point. Your PSA nadir is the smallest PSA value recorded since your radiation finished, measured in nanograms per millilitre (ng/mL). It is not a target anyone sets for you in advance. It is a number your team can only identify looking backwards, once your PSA has stopped falling and settled. Because radiation treats the prostate in place rather than removing it, healthy prostate tissue keeps making a small amount of PSA, so the nadir after radiation is usually a low but still detectable figure rather than zero. Your treating team reads your own nadir alongside your full history.
What is a good PSA nadir value after radiation?
There is no single universal cut-off. Many radiation oncologists describe a nadir under about 0.5 ng/mL as a reassuring result, and a lower nadir is generally viewed more favourably than a higher one. But the figure your own team considers good depends on your risk group, the radiation technique used, and whether hormone therapy formed part of your plan, since hormone therapy pushes PSA down while it is active. The direction and stability of your PSA over several readings matters more than any one value. Only your treating team can say what your own nadir means for you.
What is the Phoenix definition of biochemical recurrence?
The Phoenix definition is the rule most teams use to decide when a PSA rise after radiation counts as a biochemical recurrence. It sets the threshold at a rise of 2 ng/mL or more above your nadir, which is why it is often written as nadir plus 2. It was agreed at the 2005 RTOG-ASTRO consensus conference held in Phoenix, Arizona, and it replaced an older 1996 rule based on three consecutive rises. The threshold was deliberately set high enough that a temporary PSA bounce is not mistaken for treatment failure. Reaching that threshold prompts further assessment, not an automatic conclusion.
How long does it take to reach PSA nadir after radiation?
Slowly, and that is expected. Unlike surgery, where PSA falls within weeks, PSA after radiation drifts down gradually as treated cells die off over months. It commonly takes around 18 months to three years to reach nadir, and for some men it takes longer still. If hormone therapy was part of your plan, your PSA may fall very low early on and then settle at a different level once that therapy ends. A slow decline is not a sign that something has gone wrong. Your team tracks the trend across repeat tests rather than reacting to a single reading.
Why does my PSA not drop to zero after radiation?
Because your prostate is still there. Radiation is aimed at the cancer within the gland, but normal prostate tissue remains in place and normal tissue makes PSA. That is the key difference from a radical prostatectomy, where the gland is removed and PSA is expected to become undetectable. After radiation, a small persistent number is the expected outcome, not a sign that treatment failed. What your team watches is whether that number stays low and stable over time, or begins to climb. A detectable nadir on your report is not, on its own, a cause for alarm.
Does a rise above my nadir always mean the cancer has come back?
No. A single rise above your nadir is common and often temporary. A PSA bounce, a brief rise that settles again without any treatment, is well recognised after radiation and is seen more often after brachytherapy and in younger men. It typically appears within the first couple of years and then falls back. This is precisely why the Phoenix threshold was set at nadir plus 2 rather than at any rise at all. If your PSA does cross that threshold, your team will repeat the test and look at the wider picture before drawing any conclusion about your specific case.
This page explains general PSA follow-up terminology after radiation therapy. It is not a substitute for your own treating team’s interpretation of your specific report, diagnosis and treatment plan.