The purpose of this study is to determine the efficacy and safety of a personalised surveillance and intervention protocol for patients with familial adenomatous polyposis (FAP) that have undergone (procto)colectomy.

Familial adenomatous polyposis (FAP) is characterized by formation of up to hundreds to thousands of polyps throughout the entire colon and rectum. When left untreated, nearly all patients with FAP develop colorectal cancer at a median age of 35-45 years. To prevent colorectal cancer in patients with FAP, prophylactic colorectal surgery is performed. The preferred surgical procedures for FAP are a restorative proctocolectomy with ileal pouch-anal anastomosis (IPAA) or a subtotal colectomy with ileorectal anastomosis (IRA) or ileosigmoidal anastomosis (ISA).

After both types of prophylactic colorectal surgery, subtotal colectomy with IRA/ISA or proctocolectomy with IPAA, patients will require life-long surveillance because disease progression and development of new adenomas in retained rectum, pouch or residual rectal cuff will occur.

The 10-years risk of developing one or more adenomas in the rectum after IRA is 100% compared to 33% in the pouch after IPAA. The risk of developing rectal cancer after IRA was found to be 9% and 11% in two large studies with a median follow-up of 12.8 and 15 years, respectively. One study showed that the 10-years risk of developing a carcinoma in the pouch was 1%. As patients are usually operated at a young age, and nowadays have a long life-expectancy, the actual cumulative life-time risk will presumably be higher.

The recently published ESGE (European Society of Gastrointestinal Endoscopy) polyposis guideline recommends a one to two yearly endoscopic surveillance interval after prophylactic colorectal surgery in FAP, both for patients that underwent IRA/ISA and IPAA, with removal of all polyps >5mm. This recommendation is based on expert-opinion, since no studies have been reported comparing the efficacy and safety of different surveillance intervals. No advices are provided on which patients will benefit from which surveillance interval.

With the proposed study, the investigators aim to provide evidence for personalized endoscopic surveillance for patients with FAP that have undergone (procto)colectomy with construction of an IRA/ISA or IPAA with the goal to prevent development of advanced neoplasia (AN) by endoscopically removing lesions before they progress to AN.

Inclusion Criteria:

• Diagnosis of FAP, at least one of following: genetic diagnosis (proven APC germline mutation) and/or clinical diagnosis (>100 colorectal adenomas in combination with a positive family history of FAP)
• Have undergone prophylactic (procto)colectomy with IRA/ISA or IPAA
• Age 18 years or older

Exclusion Criteria:

• Not able to remove all polyps with an indication for removal during (multiple) clearing endoscopies
• Cancer at baseline endoscopy
• Need for surgery

All patients with FAP treated at one of the participating centres.