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Prostate

What does radiotherapy for prostate cancer do to the bowel?

Radiotherapy spares continence where surgery does not. Its own cost falls somewhere else, and one part of it does not recover with time.

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Answer

Bowel function is radiotherapy's distinctive harm: in the ProtecT trial, which randomised 1,643 men and followed patient-reported outcomes for six years, bowel function was worse in the radiotherapy group at six months and then partly recovered — except for bloody stools, which became more frequent over time — while radiotherapy had little effect on urinary continence, the function surgery damaged most.

Key takeaways

  • In ProtecT, radical prostatectomy had the greatest negative effect on sexual function and urinary continence, while radiotherapy had little effect on continence.
  • Radiotherapy's distinctive harm was bowel function, worse than the other groups at six months and then partly recovered.
  • Bloody stools were the exception: they became more frequent over time rather than recovering.
  • In a Korean series of 249 men treated with CyberKnife stereotactic radiotherapy and followed for a median of 50 months, grade 2 or worse gastrointestinal toxicity occurred in 1.2 percent acutely and 0.8 percent chronically.
  • Genitourinary toxicity of grade 2 or worse in that series was 7.6 percent acutely and 6.0 percent chronically.
  • Three-year biochemical recurrence-free survival was 96.5 percent with 35 Gy in five fractions and 96.8 percent with 36 Gy in four, with no significant difference after adjustment.
  • In a Korean comparison of salvage radiotherapy after prostatectomy, four-week and six-week schedules produced no significant difference in late toxicity, and no grade 3 or worse late genitourinary toxicity occurred in either.

Men choosing between prostate cancer treatments are often told that surgery and radiotherapy have similar cancer outcomes for localised disease. That is roughly true, and it makes the side effects the deciding information rather than a footnote. The two treatments do not damage the same thing.

What the randomised evidence shows

ProtecT randomised 1,643 men with localised prostate cancer to active monitoring, surgery or radiotherapy, and — unusually — collected patient-reported function for six years rather than relying on what clinicians recorded.

Where each treatment's harm falls, ProtecT patient-reported outcomes
FunctionSurgeryRadiotherapy
Urinary continenceGreatest negative effectLittle effect
Sexual functionGreatest negative effectAffected, less so
Bowel functionLittle effectWorse at 6 months, partly recovered

The word doing the work in that last cell is 'partly'. Bowel function improved after the six-month low point but did not return to where it started, and one component moved the other way entirely.

What Korean series report

ProtecT used conventional radiotherapy schedules from the 1999-to-2009 recruitment era. Modern stereotactic delivery concentrates treatment into far fewer sessions, and Korean series report considerably lower gastrointestinal toxicity.

In 249 men treated with CyberKnife stereotactic body radiotherapy and followed for a median of 50 months, grade 2 or worse gastrointestinal toxicity was 1.2 percent acutely and 0.8 percent chronically. Genitourinary toxicity of the same grade was higher, at 7.6 and 6.0 percent. Three-year biochemical recurrence-free survival was 96.5 percent with 35 Gy in five fractions and 96.8 percent with 36 Gy in four, with no significant difference after adjustment.

A separate Korean comparison, in men having salvage radiotherapy after prostatectomy, found no significant difference in late genitourinary or gastrointestinal toxicity between a four-week and a six-week schedule, and no grade 3 or worse late genitourinary toxicity in either arm.

How to weigh the two sets of numbers

The Korean figures are single-arm series, not randomised comparisons, and clinician-graded toxicity scales and patient-reported function do not measure the same thing. A man reporting that his bowels are not what they were may score zero on a toxicity grade. The two literatures should not be merged into one reassurance.

What both support is the shape of the choice. Radiotherapy is the option that largely preserves urinary continence and charges its price to the bowel; surgery is the option that does the reverse. Which trade is acceptable is not a clinical question, and it is the one the patient is uniquely qualified to answer.

Sources

  1. 1

    Donovan JL, Hamdy FC, Lane JA, Mason M, Metcalfe C, Walsh E, et al.

    Patient-reported outcomes after monitoring, surgery, or radiotherapy for prostate cancer

    New England Journal of Medicine 2016;375(15):1425-1437

    doi:10.1056/nejmoa1606221 · PMID:27626365

    https://doi.org/10.1056/nejmoa1606221
  2. 2

    Lim J, Jo YY, Lee S, Kim YJ

    Clinical outcomes of CyberKnife stereotactic radiotherapy for localized prostate cancer: comparison of 35 Gy in 5 fractions and 36 Gy in 4 fractions

    Journal of Radiation Research 2026;67(3):402-411

    doi:10.1093/jrr/rrag017 · PMID:41978285

    https://doi.org/10.1093/jrr/rrag017
  3. 3

    Kim HU, Lee J, Jo YY, Lee S, Kim YJ

    Toxicity comparison between a four-week moderate hypofractionation and a six-week conventional fractionation in the post-prostatectomy salvage radiotherapy for prostate cancer

    Clinical Genitourinary Cancer 2026;24(1):102473

    doi:10.1016/j.clgc.2025.102473 · PMID:41387133

    https://doi.org/10.1016/j.clgc.2025.102473