Stereotactic body radiotherapy for prostate cancer delivers the full course in five fractions, relying on tight margins, careful motion management and steep dose falloff to spare the rectum and bladder. It borrows many ideas from lung SBRT — conformity, dose gradients, near-max doses — but adds one that is specific to the prostate: protecting the urethra that runs through the middle of the target.
Prostate SBRT delivers the whole course in five fractions. A common prescription, for example, is 36.25 Gy to the PTV with 40 Gy to the CTV, with at least a day between fractions. PACE-B compared SBRT with conventional or moderately hypofractionated IMRT in a randomized trial.
With so few, large fractions, a few things carry more weight than in a 20- or 39-fraction plan:
The same full-bladder, empty-rectum approach as conventional treatment, held to a tighter standard.
With tight margins, a change in filling that would be tolerable over 39 fractions can matter in one of five.
The PACE-B trial recommended a minimum bladder filling, and protocols ask for consistent bladder and rectal volumes between simulation and treatment.
Image fusion aligns an outside scan with the planning CT so the target can be drawn using information the CT doesn't show. For prostate SBRT a planning MRI is used, ideally in the treatment position on a flat couch top, with bladder and rectal preparation comparable to the CT simulation. PSMA PET may also be fused when an intraprostatic lesion is boosted.
With tight margins, and the urethra contoured as its own organ, MRI's view of the prostate edges and urethra matters even more. The registration is rigid on the prostate, guided by the fiducial markers. Differences in bladder or rectal filling between the scans change the prostate's shape and position — another reason to match the preparation.
Tracking the prostate's position during delivery — with fiducials, electromagnetic transponders, repeated imaging or MR guidance, depending on the equipment.
The prostate can drift during a fraction as the rectum and bladder fill. Monitoring shows when it has moved beyond tolerance.
What counts as out of tolerance, and what to do about it, is set by your department's SBRT policy.
A gel placed between prostate and rectum.
Extra separation makes the steep dose falloff more effective at sparing the rectal wall.
Volumes and margins come from your protocol.
The prostate, sometimes with the proximal seminal vesicles.
In some protocols the CTV and PTV receive different prescribed doses, so both volumes are evaluated.
A small expansion for residual setup and motion uncertainty.
SBRT margins are smaller than conventional ones because motion is monitored more closely; that is what makes the rectal and bladder constraints achievable.
The prostatic urethra, often with a small radial expansion, contoured so its maximum dose can be limited. More →
Controlling urethral hotspots is one of the main levers on urinary toxicity in SBRT.
Click an organ for its anatomy, why it matters and contouring resources. Superscripts link to the study behind each value.
| Organ | 36.25 Gy / 5 fxTo PTV (40 Gy to CTV) |
|---|---|
| RectumIdeal limits; applied to rectum and sigmoid. | |
| Bladder | |
| UrethraApplied to the urethral PRV (urethra + 0.2–0.3 cm). | |
| Femoral Heads |
|
| Small Bowel | |
| Large Bowel / SigmoidGeneral 5-fraction SBRT limits, not prostate-specific. | |
| Penile Bulb |
Values quoted from eviQ 4140 (last reviewed 7 November 2024; review due 31 December 2026) and CORSAIR, checked by DoseLab on 2026-09-22. Verify against your protocol, institutional policy and physician directive before clinical use.
Randomized phase 3 trial of SBRT versus conventionally or moderately hypofractionated IMRT for localized prostate cancer; acute toxicity report. Later reports give 2-year toxicity and 5-year outcomes.
Published protocol of a TROG phase 3 trial of prostate SBRT with androgen deprivation. Citation as given in the eviQ reference list; not yet checked against the journal.
Prospective trial of prostate SBRT for low-risk disease with long-term follow-up. Citation as given in the eviQ reference list; not yet checked against the journal.
Citation as given in the eviQ reference list; not yet checked against the journal.
Practical guidance on visualizing the urethra for prostate planning. Citation as given in the eviQ reference list; not yet checked against the journal.
RTOG consensus panel atlas of male and female pelvic normal tissues. Citation as given in the eviQ reference list; not yet checked against the journal.