One of the most common questions I hear in clinic after completing prostate cancer radiotherapy is: “How will we know if it’s worked?” It’s an entirely reasonable thing to want to know, and we use a blood test called the PSA — Prostate-Specific Antigen — to monitor your response over time. PSA is a protein produced by prostate tissue, and in men who have had radiotherapy for prostate cancer, we expect the PSA to fall gradually over the months following treatment. Unlike surgery, where the prostate is removed and PSA should drop to near-zero relatively quickly, radiotherapy works by damaging tumour cells in a way that causes them to die off over time. This means the PSA decline after radiotherapy is slower and more gradual — sometimes taking 18 months or more to reach its lowest point, which we call the nadir. It’s important to understand this from the outset, because a PSA that hasn’t dropped immediately after finishing treatment does not mean the treatment hasn’t worked. Sometimes, this effect is overridden by the fact that you are receiving hormone therapy alongside the radiotherapy, in which case, the hormone treatment will likely already have suppressed your PSA. In these cases, it will only be after the hormone therapy completes when you will know whether the radiotherapy has been successful.
The most widely accepted benchmark for defining treatment success — and unfortunately, for defining recurrence — after radical radiotherapy is something called the Phoenix Criteria, originally established following a major consensus statement to standardise comparisons in clinical trials, and now widely adopted in clinical practice. Put simply, this defines a rise in PSA to 2 ng/mL above the lowest point ever reached as the threshold for concern. So if your PSA nadir after treatment is 0.3 ng/mL, a PSA rising to 2.3 ng/mL would prompt further discussion and investigation. A single elevated reading rarely triggers immediate treatment. We look at the trend, the rate of rise, and the clinical context. The picture is different again for men who have had surgery followed by salvage radiotherapy, where we expect the PSA to be considerably lower throughout, and where the thresholds we use shift accordingly.
What I consistently see in clinic is that PSA monitoring, though undeniably useful, brings with it real psychological weight. The weeks leading up to a blood test and the days waiting for a result can feel disproportionately anxious relative to the clinical significance of the number itself. This is entirely understandable, and it is something I take seriously in every consultation. We are not just tracking a number — we are supporting a person through ongoing uncertainty. If you find the surveillance process difficult, please say so. It is not a sign of weakness; it is an appropriate response to a stressful situation, and there are things we can do to help, whether that is adjusting the frequency of tests, providing results more promptly, or connecting you with specialist support. Macmillan Cancer Support and Prostate Cancer UK both publish excellent patient resources on PSA monitoring that you may find reassuring to read between appointments.