LETTER TO THE EDITOR Active ulcerative colitis diagnosed by 18 F-FDG PET/CT in an anti-TNF alpha treated patient with no visible luminal lesions on colonoscopy Peter Parbo & Katrine Stribolt & Charlotte Siggaard Rittig & Lars Christian Gormsen Accepted: 3 February 2014 # Springer-Verlag Berlin Heidelberg 2014 Dear Editor: We would like to draw attention to some of the diagnostic difficulties associated with the increasing use of anti-tumor necrosis factor (anti-TNF) drugs to treat inflammatory bowel disease. Ulcerative colitis lesions are mainly localized in the muco- sal surface of the colon extending from the rectum proximally through the colon. Colonoscopy with biopsies is considered the gold standard diagnosing active ulcerative colitis. However, patients treated with the anti-TNF alpha drug, adalimumab, show more mucosal healing than patients receiv- ing placebo [1], rendering direct visualization of lesions by colonoscopy difficult. A 16-year-old boy with known ulcerative colitis was hos- pitalized in our institution due to prolonged fatigue, fever, and chest pain. Months prior to the patients’ hospitalization, treat- ment with adalimumab had been initiated due to frequent relapses of active disease. On admission, two to four bloodless defecations per day were recorded, and no signs of disease activity were evident at colonoscopy. Computed tomography showed only discretely thickened walls of the colon, and small bowel capsule endoscopy showed no signs of inflammation. Due to deteriorating overall clinical status, an 18 F-FDG PET scan was performed revealing avid 18 F-FDG uptake in the entire colon. An additional colonoscopy only indicated light disease activity inconsistent with the clinical presentation. However, the patient’ s clinical condition continued to deteriorate and based on the PET scan results, it was decided to perform a total colectomy. Subsequent pathological examination of the colon showed mul- tiple crypt abscesses under a healed mucosa. The patient soon recovered and was discharged. As reported by Lapp et al. [2], signs of active ulcerative colitis may be absent during colonoscopy while evident as avid colonic 18 F-FDG uptake on an 18 F-FDG PET scan. There may be several explanations for this discrepancy. First, performing a colonoscopy is often challenging due to stric- tures and unreachable segments. Second, the disease some- times mainly affects deeper parts of the bowel wall and may, thus, not be visible from the luminal side of the colon. 18 F-FDG PET/CT is a noninvasive modality visualizing all layers of the colon and is not hampered by the same proce- dural and anatomical limitations. Reflecting these characteris- tics, sensitivity of 18 F-FDG PET/CT to detect active disease in adults with ulcerative colitis is 95.8 % and 81.3 % in patients with Crohn’ s disease [3]. In children, the sensitivity of 18 F- FDG PET is 89.5 % in diagnosing ulcerative colitis and slightly lower (81.3 %) in Crohn’ s disease [4]. Thus, it is by now well documented that 18 F-FDG PET has an excellent diagnostic performance in adults and is also very useful in children where colonoscopy is best avoided. Mucosal healing is an important predictor of long- and short- term outcomes in ulcerative colitis. As demonstrated by Sandborn et al. [1], adalimumab induces mucosal healing and is therefore increasingly used to treat the disease. However, while mucosal healing is associated with a better prognosis, it does not always entail complete healing of deeper inflamma- tion. This was most likely true in the current case, where an apparently healed mucosal surface hid deeper crypt abscesses. P. Parbo : L. C. Gormsen Department of Nuclear Medicine & PET Center, Aarhus University Hospital, Nørrebrogade 44, 8000 Aarhus C, Denmark K. Stribolt Department of Histopathology, Aarhus University Hospital, Nørrebrogade 44, 8000 Aarhus C, Denmark C. S. Rittig Department of Pediatrics, Aarhus University Hospital, Skejby, 8200 Aarhus N, Denmark L. C. Gormsen (*) Department of Nuclear Medicine & PET Center, Skejby, Aarhus University Hospital, 8200 Aarhus N, Denmark e-mail: lars.christian.gormsen@ki.au.dk Int J Colorectal Dis DOI 10.1007/s00384-014-1840-z