This international, multi-centre randomised controlled trial will compare dye-based chromoendoscopy with virtual chromoendoscopy, using NBI/BLI, for adenoma detection during routine surveillance pouchoscopy in adults with familial adenomatous polyposis (FAP) and an ileal pouch-anal anastomosis (IPAA). The estimated study duration is 2 years. Participants will undergo their usual scheduled pouchoscopy, performed by endoscopists experienced in FAP. Before the procedure, they will be randomised 1:1 to dye-based or virtual chromoendoscopy. Adenomas requiring endoscopic treatment will be removed during the same procedure according to standard practice.

To our knowledge, no previous study has directly compared these techniques in this setting. Current guidelines recommend surveillance in patients with FAP and a pouch and permit dye-spray chromoendoscopy, but do not specify whether dye-based or virtual chromoendoscopy should be preferred. Practice varies between centres: virtual chromoendoscopy is commonly used at St Mark's Hospital, while some European centres primarily use dye-based chromoendoscopy. This study therefore aims to standardise practice and generate evidence to inform future surveillance strategies.

Eligible patients will be adults aged ≥18 years with FAP, defined by a proven APC germline mutation or a clinical diagnosis of >100 colorectal adenomas with a positive family history, and who have undergone IPAA after primary proctocolectomy or secondary proctectomy following IRA/ISA. Patients will be identified through routine endoscopy booking systems. Those who have consented to email communication from the Polyposis Registry team will receive a Participant Information Sheet in advance. They will be approached again on the day of their procedure, given the opportunity to ask questions, and consented before randomisation. Patients who do not consent will undergo their planned pouchoscopy as normal, without study randomisation. Patients lacking capacity to consent will not be approached.

Randomisation will be performed using an independent computer-generated programme within Castor EDC, with allocation in a 1:1 ratio. Block randomisation will ensure balanced distribution between arms within each centre, and stratification by centre will account for differences in patient characteristics and local practice. Blinding is not feasible because dye-based and virtual chromoendoscopy have visually distinct appearances.

During pouchoscopy, the pre-pouch ileum, pouch body and rectal cuff will be carefully inspected. In the dye-based arm, indigo carmine will be applied using a spray catheter before withdrawal and mucosal inspection. In the virtual chromoendoscopy arm, inspection will be performed using NBI/BLI according to local platform availability. The endoscope will be advanced to the pre-pouch ileum, followed by systematic withdrawal and spiral mucosal inspection. Lesions will be documented by size and location, including pouch body and rectal remnant/rectal cuff, using a polyp burden scoring table. Retroflexion will be performed to assess the rectal cuff. Polyp size will be estimated in millimetres, supported where appropriate by biopsy forceps of known size. Adenomas will be resected using standard polypectomy techniques where indicated, including polyps >5 mm in the pouch body, >2 mm in the rectal cuff, or lesions suspicious for high-grade dysplasia or early cancer.

Because assessment of small polyps can vary between endoscopists, particularly for lesions <5 mm, endoscopic images will be used to assess inter-rater and intra-rater reliability for polyp burden and size. If reliability is acceptable, smaller polyps will be included and reported.

Quality parameters will be collected for each procedure, including adjusted Boston Bowel Preparation Scale assessment for the pouch and total procedural time. Procedural time will include scope introduction, irrigation, dye application where applicable, withdrawal and inspection, retroflexion, and removal and retrieval of polyps.

Post-procedure follow-up will follow routine care. Patients will be asked to monitor for adverse events after pouchoscopy and contact the hospital if needed. Future surveillance will be scheduled according to each centre's usual policy.

Study data will be held in routine hospital systems accessible to the patient's usual clinical team and in a study file. Participants will be assigned a study number. No identifiable patient information will leave the Trust or be accessible to anyone outside the usual care team. Anonymised data will be entered into Castor EDC. Pseudonymised data will be transferred to the central study team at Amsterdam University Medical Center under an existing data transfer agreement for pooled analysis.

The study does not expose participants to risks beyond routine surveillance pouchoscopy. There is no direct individual benefit but findings may benefit future patients with FAP and families by improving evidence.