Definitive external beam radiotherapy treats the prostate — and, depending on risk, the seminal vesicles and pelvic lymph nodes — with IMRT or VMAT, most often in 20 or 39–40 fractions. The planning challenge is anatomical: the rectum lies directly behind the target and the bladder sits above and in front, so plans are judged as much by their rectal and bladder DVHs as by target coverage.
This page covers setup, target volumes, organs at risk with cited constraints, and plan evaluation. The numbers — prescriptions, margins and constraints — belong to the protocol you’re following.
The prostate moves with bladder and rectal filling. Setup is about making each day's anatomy look like the planning CT.
A drinking protocol so the bladder is comfortably full — and similarly full — at simulation and every treatment.
A fuller bladder lifts more of the bladder wall and small bowel away from the high-dose region. Just as important is consistency: a bladder much fuller or emptier than at simulation shifts the prostate and changes the dose to both organs.
Protocols ask for a comfortably full bladder and an empty rectum, with consistent volumes between simulation and treatment. How much and how long before is set by department policy.
Measures to keep the rectum empty and consistent — diet advice, micro-enemas or laxatives, depending on the department.
Gas and stool in the rectum can push the prostate forward and change its position from day to day. A distended rectum at simulation can also lead to a plan that doesn't match treatment days.
If the rectum is distended on the planning CT, many departments re-simulate rather than plan on unrepresentative anatomy.
Small radiopaque markers implanted in the prostate so its position can be seen on daily imaging.
The prostate moves independently of the pelvic bones. Matching on markers (or directly on soft tissue) targets the prostate itself rather than the bones around it.
Check that markers have not migrated between simulation and treatment, and know whether your department matches on markers, soft tissue or both.
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 this is mainly MRI — T2-weighted, with diffusion and contrast sequences when an intraprostatic lesion is to be boosted — and, where available, PSMA PET/CT. The registration is usually rigid and centered on the prostate, using the fiducial markers when they are present.
The prostate's boundaries, the apex and its edges against the rectum and penile bulb are much clearer on MRI than CT, and the urethra is contoured from MRI. MRI is particularly helpful with a hip prosthesis or a rectal spacer. A dominant lesion for a boost is identified on MRI and, if available, PSMA PET. Bladder and rectal filling change the prostate's shape and position, so the MRI is best taken with the same preparation as the planning CT.
A gel injected between the prostate and rectum to increase the distance between them for the course of treatment.
More separation means the anterior rectal wall sits further from the high-dose region. A randomized trial reported lower rectal dose and a continued benefit in rectal toxicity.
Spacers change rectal anatomy on imaging; MRI helps delineate the prostate–spacer–rectum interface.
Supine positioning with knee and ankle supports, and optionally a vacuum bag.
Reproducible leg and pelvic position keeps the femoral heads and pelvis consistent, so daily imaging corrections stay small.
What each volume represents. Which structures are included, and every margin, come from your protocol.
The prostate, with or without part or all of the seminal vesicles depending on risk group.
How much seminal vesicle is included is one of the main risk-based decisions in definitive prostate treatment, and it changes rectal and bladder dose considerably.
A visible tumor nodule within the prostate, usually defined on MRI or PSMA PET.
Some approaches deliver a boost to the dominant lesion. When boosting, OAR tolerance takes priority over boost coverage.
Pelvic lymph node regions treated electively in higher-risk disease. More →
Adding nodes enlarges the treated volume upward, bringing small and large bowel into play.
An expansion for setup and delivery uncertainty.
The posterior PTV margin overlaps the anterior rectal wall. How that overlap is handled — and how daily image guidance is used — largely decides the rectal DVH.
Definitive prostate radiotherapy is now delivered either in around 39–40 fractions or in 20 fractions. Randomized trials — CHHiP, PROFIT and HYPRO — underpin moderate hypofractionation.
For planners the key point is that the constraints change with the schedule. A rectal Vx written for 2 Gy fractions cannot be applied to a 3 Gy-per-fraction plan: the 20-fraction constraints below come from the CHHiP protocol, the 39–40 fraction constraints from other sources. Use the column for the schedule you are planning.
Click an organ for its anatomy, why it matters and contouring resources. Superscripts link to the study behind each value.
| Organ | 60 Gy / 20 fxModerate hypofractionation | 70 Gy / 28 fxModerate hypofractionation | 78–80 Gy / 39–40 fxConventional |
|---|---|---|---|
| RectumAcceptable limits; stricter ideal targets exist. | |||
| Bladder | |||
| Femoral Heads |
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| |
| Penile BulbNever at the expense of PTV coverage. | — |
| |
| Small Bowel | — |
| |
| Large Bowel / SigmoidGeneral conventional limits, not prostate-specific. | — |
Values quoted from eviQ 234 (last reviewed 7 November 2024; review due 31 December 2026), ASTRO DVH Compendium 2026 and CORSAIR, checked by DoseLab on 2026-10-14. Verify against your protocol, institutional policy and physician directive before clinical use.
Randomized phase 3 non-inferiority trial of conventional versus moderately hypofractionated IMRT for localized prostate cancer. A correction to this article was also published.
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.
NRG Oncology's page for RTOG 0126, a randomized dose-escalation trial for localized prostate cancer.
NRG Oncology's page for RTOG 0924, a phase 3 trial of androgen deprivation and high-dose radiotherapy with or without whole-pelvic radiotherapy.
Citation as given in the eviQ reference list; not yet checked against the journal.
A 2010 journal supplement summarizing dose-volume and outcome data for normal tissues, organized by organ.
ESTRO ACROP consensus on CT- and MRI-based prostate target delineation. 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.