Every year in the UK, more than 22,000 people receive pelvic radiotherapy, including many with prostate cancer. This study aims to understand whether the bacteria in the bowel (the gut microbiome) change during prostate radiotherapy, and whether these changes might be linked to side effects or overall health.

Researchers want to:

• Look at the types of bacteria in the stool of people having radiotherapy for prostate cancer.
• See whether these bacteria change during treatment.
• Check for related changes in blood, urine, and an optional rectal swab, which may help show what is happening inside the gut.
• Understand whether diet and quality of life relate to any of these changes.

Around 50 people with prostate cancer who are due to start radiotherapy will be invited to join over a two‑year period.

If you decide to join the study and give your consent, you’ll be asked to provide samples at several points:

• Before radiotherapy starts (baseline)
• Once a week during radiotherapy (for 4 weeks)
• 6–8 weeks after starting radiotherapy
• 12 weeks after starting radiotherapy
• 6‑month follow‑up
• 12‑month follow‑up

• Stool sample
• Blood sample (40 ml, taken by a delegated nurse or research nurse)
• Mid‑stream urine sample
• Optional rectal swab
• Quality‑of‑life questionnaire
• 4‑day food diary completed:
  • in the 4 days before your first stool sample
  • again at 6–8 weeks, 6 months, and 12 months

By understanding how the gut microbiome and other markers change during radiotherapy, researchers hope to improve future treatments and reduce side effects for people with prostate cancer.