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For prostate radiotherapy you are usually asked to empty your bladder and then drink a set amount of water before the planning scan and every treatment. A comfortably full bladder pushes the bowel away from the radiation. Some centres also ask for a small enema or laxatives to empty the back passage, but newer studies suggest this is not needed for everyone when daily scans check the position.
No guideline grades bladder or rectal preparation for prostate radiotherapy, and practice varies. Bladder: a comfortably full bladder lifts small bowel out of the pelvis and lowers bowel dose, but drinking protocols give inconsistent volumes from day to day (Nasser 2021 review). In a randomised trial of 110 men, 540 ml of water gave more reproducible bladder volumes than 1080 ml (median variation 76 against 105 ml) with no more toxicity; a 2021 meta-analysis of 11 studies (417 patients) found 300 to 400 ml gave the best reproducibility and that reproducibility fell as the volume rose. Checking the bladder on imaging before each fraction is the practical safeguard.
Rectum: a 2014 systematic review of 18 studies found no robust evidence for one rectal emptying method over another; randomised trials of simethicone (SPoRT, 30 men) and an antiflatulent (78 men) found no reduction in rectal volume or rescans; a small trial found an osmotic laxative gave less rectal gas than a bulking laxative with probiotic. In 295 Royal Marsden patients with daily cone-beam CT, micro-enema preparation reduced the share with a large rectum on the planning CT by only 10 to 15% and did not change intrafraction motion; the authors conclude preparation is not universally needed and recommend intervention where a large rectum coincides with seminal vesicle involvement or risk. A published comment reads the same data as support for risk-adapted, not universal, omission. The ESTRO ACROP IGRT guideline (2019) is cited by Alexander et al. as not recommending routine preparation; its full text could not be read.
Why: A comfortably full bladder lifts the bowel away from the radiation; a moderate drink is easier to repeat the same way every day than a large one.
“Consuming 300-400 ml of water potentially leads to the best bladder volume reproducibility; moreover, the higher the water consumption volume, the lower the bladder volume reproducibility.”
Biomedical Journal (Chen et al.) 2021 · Systematic review and meta-analysis (10 cohort studies, 1 randomised trial)
Why: Drinking more fills the bladder more on the planning day than on treatment days, so the shapes stop matching.
“The 540 mL bladder-filling arm resulted in reproducible BVs throughout a course of RT, without any deterioration in QoL or increase in toxicities for prostate patients.”
Practical Radiation Oncology (Mullaney et al.) 2014 · Randomised trial (110 patients)
Why: With a larger area treated, a full bladder matters more for keeping the small bowel out of the high-dose region.
“While treating prostate only with empty bladder could be feasible, treating the prostate and pelvic lymph nodes should be preferably done with a full bladder, to limit the radiation dose to the small bowel.”
Technical Innovations and Patient Support in Radiation Oncology (Nasser et al.) 2021 · Narrative review
Why: Even with a drinking routine the bladder fills differently on different days, and the scan shows it before the dose is given.
“CBCT prior each fraction of RT is useful for urine volume estimation in order to ensure consistent bladder filling for RT.”
Technical Innovations and Patient Support in Radiation Oncology (Nasser et al.) 2021 · Narrative review
Why: With a daily scan before each treatment, enemas changed the position of the prostate very little and only modestly reduced the number of people with a full rectum.
“Findings suggest rectal preparation is not universally required with contemporary IGRT.”
Clinical and Translational Radiation Oncology (Alexander et al., Royal Marsden) 2026 · Retrospective cohort (295 patients), single centre
Why: A changing rectum can push the seminal vesicles out of the treated area, so these patients are the ones preparation or adaptation helps.
“Interfraction changes in rectal volume can lead to significant underdosing of seminal vesicles therefore we do recommend further intervention for patients with both a large rectum on pCT and seminal vesicle involvement, or a high risk of seminal vesicle involvement.”
Clinical and Translational Radiation Oncology (Alexander et al., Royal Marsden) 2026 · Retrospective cohort (295 patients), single centre
Why: Enemas, laxatives, diet and anti-wind medicines have all been tried, and none has proved better than another.
“There is no robust evidence to recommend one rectal emptying strategy over another.”
Practical Radiation Oncology (McNair et al.) 2014 · Systematic review (18 studies)
How care is usually carried out, read from the guidelines and standards cited. Your own team's protocol and their judgement come first; this page is for understanding it, not for doing it yourself.
No guideline sets a best practice; the evidence supports a moderate, timed drink after emptying the bladder (300 to 540 ml in the trials; Chen 2021, Mullaney 2014), a full bladder when the pelvic nodes are treated, and confirmation of bladder filling on the pre-treatment scan. For the rectum, with daily 3D image guidance, routine preparation for everyone is not supported (Alexander 2026; ESTRO ACROP 2019 as cited there); preparation or plan adaptation is targeted at people with a large rectum on the planning CT and seminal vesicle involvement or risk. Where the sources differ: Alexander et al. read their data as showing preparation is not universally required, while the published comment on it reads them as support for risk-adapted omission only, pending prospective study. No method of rectal emptying is better than another (McNair 2014).
Biomedical Journal (Chen et al.) 2021 · Practical Radiation Oncology (Mullaney et al.) 2014 · Technical Innovations and Patient Support in Radiation Oncology (Nasser et al.) 2021 · Clinical and Translational Radiation Oncology (Alexander et al., Royal Marsden) 2026 · Clinical and Translational Radiation Oncology (Gupta et al.) 2026 · Practical Radiation Oncology (McNair et al.) 2014
Where it differs: Patients empty their bladder an hour before planning and treatment and drink 350 ml of water; micro-enema use depends on the prescription.
Evidence: Described as the routine for 295 patients in the Alexander 2026 cohort; within the volume range the meta-analysis found most reproducible.
Clinical and Translational Radiation Oncology (Alexander et al., Royal Marsden) 2026 · Biomedical Journal (Chen et al.) 2021
Where it differs: No set volume; the patient drinks until they feel full.
Evidence: In the meta-analysis this was less reproducible than 300 to 400 ml (mean bladder volume difference against planning of about 46 ml). The Nasser review cites a study where patients reporting a comfortably full bladder had an adequately full bladder only 76.5% of the time.
Biomedical Journal (Chen et al.) 2021 · Technical Innovations and Patient Support in Radiation Oncology (Nasser et al.) 2021
Where it differs: A portable bladder scanner measures the volume before each treatment, and the patient waits or drinks until a target is reached.
Evidence: The Nasser review cites one study where 93% of patients on a scan protocol were ready after the cone-beam CT against 75% without, and another where feedback did not improve filling; overall it judges protocols, ultrasound and biofeedback to fall short of consistent filling.
Technical Innovations and Patient Support in Radiation Oncology (Nasser et al.) 2021
Where it differs: The bladder is kept empty, in some single-fraction schedules with a catheter.
Evidence: Described as feasible for prostate-only treatment in the Nasser review, citing a non-inferiority report; not recommended when the pelvic nodes are treated.
Technical Innovations and Patient Support in Radiation Oncology (Nasser et al.) 2021
Where it differs: Patients take simethicone during treatment to reduce rectal gas.
Evidence: Two randomised trials found no overall reduction in rectal volume, gas or planning rescans (78 and 30 patients); SPoRT found less rectal volume variability from week three onwards with simethicone, which its authors say needs a larger study.
Practical Radiation Oncology (McGuffin et al.) 2018 · Journal of Medical Imaging and Radiation Oncology (Ward et al.) 2022
Where it differs: Patients follow a low-gas diet and take a laxative during treatment.
Evidence: A trial of 17 patients found an osmotic laxative gave lower rectal gas than a bulking laxative with probiotic (OR 3.2 for a higher gas rating with the bulking regimen); the authors call for larger studies.
The bladder and rectum change size from day to day and move the prostate with them. A randomised trial found that drinking about half a litre of water before each treatment kept the bladder more consistent than drinking a litre, and small trials of anti-wind medicines before prostate radiotherapy found no effect.
A spacer injected between the prostate and the rectum pushes the bowel out of the high-dose area for a few months and then dissolves. Randomised trials found fewer bowel side effects, but the procedure carries its own rare serious complications, and England's guidance allows it only with special arrangements for consent and audit.
| Recommendation | Grade, as printed | Source |
|---|---|---|
| 540 ml gave less bladder volume variation than 1080 ml “There was significantly less BV variation in the 540 mL arm when compared with 1080 mL (median: 76 mL vs 105 mL, P = .003).” | Randomised trial (110 patients) | Practical Radiation Oncology (Mullaney et al.) 2014 |
| Rectal preparation reduced large rectum on planning CT by only 10 to 15% “Results do not justify blanket use of rectal preparation for individuals undergoing radiotherapy to their prostate.” | Retrospective cohort (295 patients) | Clinical and Translational Radiation Oncology (Alexander et al., Royal Marsden) 2026 |
| The findings support risk-adapted, not universal, omission of rectal preparation “The findings are therefore best viewed as supporting risk-adapted deimplementation rather than universal omission of rectal preparation.” | Published correspondence | Clinical and Translational Radiation Oncology (Gupta et al.) 2026 |
| No consensus on bladder filling protocols “There is no consensus on what bladder filling protocols should be used for prostate external beam radiotherapy.” | Narrative review | Technical Innovations and Patient Support in Radiation Oncology (Nasser et al.) 2021 |
Shares In-room imaging for radiotherapy (cone-beam CT, ExacTrac, CT-on-rails, HyperSight), Radiotherapy, IMRT / IGRT (modern external beam).
Shares An absorbable gel or balloon between prostate and rectum before radiotherapy, used under audit, A measured drink of water before pelvic radiotherapy, and bowel preparation kept to what trials support, How many radiotherapy visits: the prostate fractionation trials, IMRT / IGRT (modern external beam).
Shares In-room imaging for radiotherapy (cone-beam CT, ExacTrac, CT-on-rails, HyperSight), Radiotherapy, IMRT / IGRT (modern external beam).
Shares In-room imaging for radiotherapy (cone-beam CT, ExacTrac, CT-on-rails, HyperSight), Radiotherapy, IMRT / IGRT (modern external beam).
Shares Prostate cancer.
Shares An absorbable gel or balloon between prostate and rectum before radiotherapy, used under audit, A measured drink of water before pelvic radiotherapy, and bowel preparation kept to what trials support, IMRT / IGRT (modern external beam), Prostate cancer.
Shares Prostate cancer.
Shares In-room imaging for radiotherapy (cone-beam CT, ExacTrac, CT-on-rails, HyperSight), Radiotherapy.