The Verdict on Tumor Volume: Can a 3-Month PET Scan Predict Who Survives Advanced Prostate Cancer?

"Ladies and gentlemen of the jury, we're here today to determine whether a treatment is actually working - and we need to reach a verdict fast." That's essentially what oncologists face every time they start a patient on a new therapy for metastatic castration-resistant prostate cancer (mCRPC). The prosecution argues: just check PSA levels, the classic blood test we've relied on for decades. The defense counters: PSA is a notoriously unreliable witness - sometimes it drops while tumors keep growing, sometimes it rises while treatment is secretly winning. Now, a new star witness has taken the stand, and its testimony might change everything: the PSMA-PET scan at the 3-month mark.

The Case File: ENZA-p and the Magic of Molecular Imaging

The ENZA-p trial (ANZUP1901) is like the Breaking Bad of prostate cancer clinical trials - it keeps revealing new layers every season. Originally designed to test whether adding radioligand therapy ([177Lu]Lu-PSMA-617) to enzalutamide could improve outcomes for men with mCRPC at high risk of early treatment failure, the trial already showed an impressive overall survival boost: 34 months for the combo versus 26 months for enzalutamide alone (Hofman et al., Lancet Oncol 2024; ASCO GU 2025 OS data). Think of it as the Avengers: Endgame of mCRPC - sometimes you need two heroes teaming up to win the fight.

But the latest sub-study, published in European Urology by Emmett and colleagues, asks a different question entirely: can the total tumor volume measured on a PSMA-PET scan at 3 months tell us who's going to make it and who isn't? (Emmett et al., Eur Urol 2026; DOI: 10.1016/j.eururo.2026.03.026)

What Even Is PSMA-TTV? (No, It's Not a Streaming Service)

PSMA-PET works like a GPS tracker for prostate cancer cells. Prostate-specific membrane antigen (PSMA) sits on the surface of most prostate cancer cells like a neon "WE'RE OPEN" sign. A radioactive tracer locks onto that sign, lighting up every tumor deposit in the body. Total tumor volume (TTV) is exactly what it sounds like - add up all those glowing spots, measure their volume, and you get a single number that captures your total cancer burden. It's like checking the box office receipts for every theater showing the same terrible movie - you want that number going down.

The Numbers That Made Oncologists Sit Up Straighter

Here's where it gets Squid Game-level dramatic. Of the 152 patients who had a PSMA-PET at 3 months:

• Any increase in tumor volume at 3 months was associated with a hazard ratio of 2.52 for death compared to any decrease. Two-year survival? Just 30% for the tumor-grew group versus 67% for the tumor-shrank group (p < 0.0001).

• Residual tumor volume above the median (103 mL) at 3 months was even more telling: HR of 3.76 for death, with 2-year survival of 34% versus 76% (p < 0.0001).

• And here's the kicker worthy of a Succession plot twist - this held true regardless of treatment arm and PSA response. Even when PSA was doing its best poker face, the PET scan saw through the bluff.

This last point matters enormously. Prior research has shown that roughly 31% of patients with a greater than 50% PSA decline still show disease progression on PSMA-PET (Gafita et al.). PSA, it turns out, is the unreliable narrator of cancer biomarkers - right up there with every character in Gone Girl.

Why This Is a Big Deal (Beyond the Cool Science)

Right now, monitoring mCRPC treatment feels a bit like reviewing a movie based solely on its trailer - you get some information, but you might be completely wrong about the actual product. PSMA-PET total tumor volume gives clinicians something closer to reading the full script. The phase 3 PSMAfore trial has already established [177Lu]Lu-PSMA-617 as a legitimate treatment option in the taxane-naive mCRPC space (de Wit et al., Lancet 2024), and PSMA-PET-derived parameters like organ-specific tumor volumes are emerging as powerful prognostic tools across disease settings (Seifert et al., Radiology 2023).

What Emmett's team has added is a potential early checkpoint - a 3-month readout that could help doctors decide whether to stay the course or change the channel. In a disease where median overall survival was 27 months in this cohort, getting actionable information at month 3 is like finding out in episode 2 of a limited series whether the show is worth finishing. Spoiler: you want those tumor volumes trending downward.

What Comes Next

This is still a phase 2 sub-study, so it needs prospective validation before anyone rewrites the treatment playbook. But the signal is strong, consistent, and - critically - independent of PSA, which means PSMA-PET isn't just duplicating information we already have. It's adding a whole new subplot that PSA can't tell you about.

For patients with mCRPC, the future might involve routine interim PSMA-PET scans serving as early verdict readers - determining whether a therapy is working well before the traditional evidence rolls in. And in the courtroom of cancer treatment, an early, accurate verdict could mean the difference between years of quality life and wasted time on ineffective therapy.

The jury is still deliberating on the final guidelines, but this evidence? It's pretty compelling testimony.

References:

1. Emmett L, Papa N, Sartor O, et al. Prognostic Value of Interim PSMA-PET Total Tumor Volume for Overall Survival Within ENZA-p, A Randomized Phase 2 Trial of Enzalutamide Versus Enzalutamide Plus [177Lu] Lu-PSMA-617 (ANZUP1901). Eur Urol. 2026. DOI: 10.1016/j.eururo.2026.03.026

2. Hofman MS, Emmett L, Sandhu S, et al. [177Lu]Lu-PSMA-617 plus enzalutamide in patients with metastatic castration-resistant prostate cancer (ENZA-p): an open-label, multicentre, randomised, phase 2 trial. Lancet Oncol. 2024. PMID: 38621400

3. de Wit R, Kulkarni HR, Engelbrecht MJ, et al. 177Lu-PSMA-617 versus a change of androgen receptor pathway inhibitor therapy for taxane-naive patients with progressive metastatic castration-resistant prostate cancer (PSMAfore): a phase 3, randomised, controlled trial. Lancet. 2024. PMID: 39293462

4. Seifert R, Emmett L, Gafita A, et al. A Prognostic Risk Score for Prostate Cancer Based on PSMA PET-derived Organ-specific Tumor Volumes. Radiology. 2023;307(4):e222010. PMID: 37070991

5. Hofman MS, Sandhu S, Emmett L, et al. Overall survival and quality of life with [177Lu]Lu-PSMA-617 plus enzalutamide versus enzalutamide alone in metastatic castration-resistant prostate cancer (ENZA-p). J Clin Oncol. 2025;43(5 suppl):17. DOI: 10.1200/JCO.2025.43.5_suppl.17

Disclaimer: The image accompanying this article is for illustrative purposes only and does not depict actual experimental results, data, or biological mechanisms.