Background <p>Acute radiation-induced diarrhoea (RID) typically develops from gut mucosa inflammation within 2 weeks of external beam radiation therapy (EBRT) to the pelvis. The Common Terminology Criteria for Adverse Events (CTCAE) commonly assesses RID. In patients with prostate cancer undergoing curative EBRT, acute RID was evaluated using both a clinician-reported outcome (ClinRO)-CTCAE, and a patient-reported (PRO) stool diary.</p> Methods <p>During 2022–2024, 40 newly diagnosed patients with prostate cancer receiving curative EBRT were enrolled at the Oncology Departments in two centres. CTCAE was assessed at baseline (week 1), at end of EBRT (week 8 or 9), 2 weeks after end of EBRT (week 10 or 11), and 8 weeks after end of EBRT (week 16 or 17). Simultaneously, patients maintained a daily PRO diary from baseline to week 10, and at week 16. Patients’ registration of stool frequency was converted to grades according to the CTCAE criteria.</p> Results <p>The clinicians assessed all 40 patients with RID Grade 0/1 using CTCAE, whereas the PRO stool diary showed 29/40 with Grade 0/1, 6/40 with Grade 2, and 5/40 with Grade 3. Grade 2 appeared in week 2 and Grade 3 in week 4. There was a significant difference (<i>p</i> &lt; 0.001) between CTCAE and PRO grades from baseline to week 16 or 17. No correlation was observed between irradiation data and RID Grades 2/3. Significant weight loss differences were noted from baseline to week 8 or 9 between patients with RID Grades 0/1 and those with Grades 2/3.</p> Conclusion <p>According to the PRO stool diary, one in four patients with prostate cancer undergoing curative EBRT experienced acute RID Grade 2/3, but none with ClinRO CTCAE. A PRO stool diary effectively monitored the onset, course of acute RID, and body weight changes. The CTCAE does not adequately assess acute RID and needs PRO stool diary supplementation.</p> Trial registration <p>Clinicaltrials.gov, NCT05684432. Registered 5 January 2023.</p>