Quantifying The Role of Urethra Sparing in Prostate SBRT Treatments
Every physicist and dosimetrist learns the rule early: keep the urethral dose down. We watch D0.1cc, Dmax, and D90 like hawks, push back on hot spots that creep too close to the catheter path, and ask for another optimization pass when a plan looks "urethra-hot." But how often do we stop to ask why — and what does that extra 15 minutes of replanning actually buys the patient sitting on the table next week?
The literature has an answer, and it's more specific than "less dose is better." There is a measurable, quantifiable relationship between the dose we deliver to the urethra and the probability that a patient will develop a urinary stricture, chronic dysuria, or worse. Understanding that relationship doesn't just make us better planners — it gives us an ‘in our bones’ feeling as to why and when we should push the dose.
The dose-toxicity relationship, in numbers
A 2022 combined analysis of 23 prospective SBRT trials by Leeman and colleagues put a number on something most of us untuitively suspected: urethral dose correlates strongly, and linearly, with urinary toxicity. Their model found that every 1 Gy increase in maximal urethral dose corresponds to roughly a 0.8% increase in acute grade ≥ 2 genitourinary toxicity and a 1% increase in late grade ≥2 toxicity.
This is the number worth carrying into every plan review: a few Gy of urethral sparing isn't cosmetic. It's a direct, quantifiable subtraction from a real patient's risk of long-term urinary morbidity.
Two strategies, two philosophies
The 2023 systematic review by Le Guevelou and colleagues in World Journal of Urology organized the field's approach to urethra sparing into two distinct techniques, and it's worth knowing the difference because they solve slightly different problems.
Urethra-steering restricts hotspots to the urethra while still treating it to (or near) the prescription dose — useful when a dominant intraprostatic lesion sits close to the urethra and needs a simultaneous integrated boost (SIB).
Urethra dose-reduction deliberately prescribes a lower dose to a planning organ-at-risk (OAR) volume around the urethra than to the rest of the gland — a more aggressive sparing strategy, best suited when the tumor isn't near the transition zone.
Both approaches work. The review's headline numbers make the case:
Urethral Dmax constraint ≤ 90 Gy EQD2 resulted in mild late grade ≥ 2 GU toxicity of 12.1–14%.
For “Urethral dose-reduction” techniques, keeping urethral dose < 70 Gy EQD2 further reduced late GU grade 2 toxicity (< 8%).
Trials that held the line at 90 GyEQD2 or below reported no cases of urethral stenosis. Trials with looser constraints reported not just higher rates of grade 2 toxicity, but severe complications — urethral strictures, and in one case, urethritis severe enough to require cysto-prostatectomy.
That's the range most of us are actively fighting for during optimization. The difficulty arises when the
Some trials pushed further. A phase II multicenter trial by Zilli and colleagues reduced urethral PRV dose to roughly 74 GyEQD2 and, at more than 70 months of follow-up, reported 5-year grade ≥2 GU toxicity-free survival around 76% — while still achieving 5-year biochemical control exceeding 90%. The review's conclusion states it plainly: dose reduction below 70 GyEQD2 may further cut long-term GU toxicity in selected patients whose tumor isn't near the transition zone, without sacrificing cancer control.
It's not just the intraprostatic urethra
Most of our attention (and most protocol constraints) focus on the intraprostatic urethra — the segment inside the D90/D98 volume we contour on the planning CT. But the review flags a subtler point that's easy to miss: the bulbo-membranous urethra, which sits below the prostate apex, has repeatedly shown up in brachytherapy and voxel-based EBRT analyses as an independently radiosensitive structure, associated with stricture risk even at low-to-intermediate doses. Bladder trigone dose has separately been linked to acute urinary retention and long-term voiding symptoms in multiple voxel-based studies cited in the review.
In practice, this means urethra sparing isn't finished the moment D0.1cc clears the intraprostatic constraint. A plan that looks clean on the standard urethra metric can still be delivering meaningful dose to structures just outside the PTV that carry their own toxicity signal — a good argument for looking at the full length of the urinary tract on a plan, not just the segment inside the prostate.
The picture in 2025: PACE-C
It's worth grounding all of this in the most current phase 3 evidence. The PACE-C trial, published in The Lancet Oncology in 2025, randomized 1,192 men with intermediate- and high-risk prostate cancer to moderately hypofractionated radiotherapy (60 Gy/20fx) versus SBRT (36.25 Gy/5fx) — notably without urethra-specific sparing protocols or rectal spacers, and with a larger treated volume than earlier PACE trials due to seminal vesicle inclusion.
The result: early CTCAE grade ≥2 genitourinary toxicity was numerically higher after SBRT (34% vs. 28%, p=0.050) — a difference that sat right at the edge of statistical significance. Gastrointestinal toxicity was more clearly elevated with SBRT (17% vs. 10%, p=0.001).
PACE-C is a reminder of what happens in a large modern trial without the kind of urethra-focused optimization the sparing literature is built around. It's not a criticism of the trial's design — its primary question was fractionation, not sparing technique — but it does throw the sparing data into sharper relief. When a trial doesn't specifically prioritize urethral dose reduction, GU toxicity trends upward. When trials do, and hold Dmax under 90 GyEQD2, that number comes down into the 12–14% range. Same disease, same technology era, meaningfully different toxicity profile — and the variable that moves is largely in our hands.
Why this matters at the workstation
None of this is meant to turn plan review into a statistics lecture. It's meant to answer the question a junior dosimetrist might reasonably ask when told to push a hotspot off the urethra for the fourth time on a plan: does this actually matter, or is it just protocol?
It matters. Based on the pooled trial data:
Staying under a 90 GyEQD2 urethral maximum keeps late grade ≥2 GU toxicity in the 12–14% range, with essentially no reported strictures.
Crossing above 100 GyEQD2 roughly doubles that toxicity rate and introduces a real risk of severe, life-altering complications.
Every additional Gy above baseline carries a quantifiable, non-trivial cost — about 1% added risk of late toxicity per Gy, according to the largest combined analysis available.
That's the biological weight behind the metric on your screen. The extra optimization pass isn't process for its own sake — it's the difference between a patient who sails through follow-up and one who ends up with a stricture requiring intervention years down the line.
References:
Le Guevelou J, Bosetti DG, Castronovo F, et al. State of the art and future challenges of urethra-sparing stereotactic body radiotherapy for prostate cancer: a systematic review of literature. World J Urol. 2023;41:3287–3299. https://doi.org/10.1007/s00345-023-04579-6
Tree AC, Hinder V, Chan A, et al. Intensity Modulated Moderately Hypofractionated Radiotherapy Versus Stereotactic Body Radiotherapy for Prostate Cancer (PACE-C): Early Toxicity Results From a Randomised Open-label Phase III Non-inferiority Trial. Lancet Oncol. 2025;26(7):936–947. https://doi.org/10.1016/S1470-2045(25)00205-0
Leeman JE, Chen Y-H, Catalano P, et al. Radiation Dose to the Intraprostatic Urethra Correlates Strongly With Urinary Toxicity After Prostate Stereotactic Body Radiation Therapy: A Combined Analysis of 23 Prospective Clinical Trials. Int J Radiat Oncol Biol Phys. 2022;112(1):75–82. https://doi: 10.1016/j.ijrobp.2021.06.037
