Peter van Mourik

122 Chapter 5 Biopsy collection Forceps biopsies and rectal suction biopsies were obtained according to detailed working instructions based on a previously published protocol 13 . In short, forceps biopsies were obtained with a flexible endoscope. The endoscope was introduced rectally, and biopsies were obtained on sight, thereby avoiding any arteries or veins. For rectal suction biopsies, patients were first treated with a sodium phosphate enema to cleanse the rectum. Next, the rectal suction device was introduced rectally, and the biopsy opening was positioned dorsolaterally and ~5 cm from the anal verge. To ensure enough material for stem cell isolation, two forceps biopsies or four rectal suction biopsies were obtained from each patient. If the biopsies were deemed to be of insufficient quality, the investigator could decide to obtain more biopsies. Storage and transport of biopsies Biopsies were stored in Ad-DF+++ with 0,1% primocin directly after the biopsy procedure and kept at 4 ° C. Detailed information on the composition and the production of this media has been previously published 13 . Within 24 hours of biopsy collection, biopsies were shipped to the central laboratory at Hubrecht Organoid Technology (HUB) in Utrecht, the Netherlands, using a courier service. Biopsies were shipped at 4 ° C. Target delivery time of samples was within 48 hours after biopsy collection. Crypt isolation and organoid generation Crypts were isolated according to previously published methods 6,13,14 . In case organoid isolation failed, this was reported to the site and the investigator had the opportunity to re-biopsy a subject. Successful organoid establishment was defined as successful crypt isolation, organoid culturing and subsequent freezing of samples for future use in the study.