Background and aims: Current guidelines suggest that routine biopsy of post-endoscopic mucosal resection (EMR) scars can be abandoned, provided that a standardized imaging protocol with virtual chromoendoscopy is used. However, few studies have examined the accuracy of advanced endoscopic imaging, such as Narrow Band Imaging (NBI) versus White Light Endoscopy (WLE) to predict histological recurrence. We aimed to assess whether NBI accuracy is superior to WLE and whether one or both techniques can replace biopsies. Patients and methods: Multicentre, randomized, patient-blinded, crossover trial, with consecutive patients undergoing the first colonoscopy after EMR of lesions ≥20mm. Computer-generated randomization and opaque envelope concealed allocation. Patients were randomly assigned to scar examination with NBI followed by WLE (NBI+WLE), or WLE followed by NBI (WLE+NBI). Histology was the reference method, with biopsies being performed in all tissues, either or not showing recurrence. Results: The study included 203 scars, 103 in group NBI+WLE and 100 in group WLE+NBI. Recurrence was confirmed histologically in 29.6% of the scars. The diagnostic accuracy of NBI was not statistically different from WLE 95% (95%CI, 92%-98%) vs. 94% (95%CI, 90%-97%); P=0.48). When assessing NBI vs. WLE, the negative predictive values were (NPV) 96% (95% CI, 93%-99%) vs. 93% (95% CI, 89%-97%), not reaching statistically significance (P=0.06). Conclusions: The accuracy of NBI for the diagnosis of recurrence was not superior to that of WLE. Endoscopic assessment of EMR scars with WLE and NBI achieved a NPV that precludes routine biopsy in cases of negative optical diagnosis.

<b><i>Introduction:</i></b> The incidence of primary colorectal lymphoma in the gastrointestinal tract is very low, the rectum being infrequently affected. The development of this entity in inflammatory bowel disease patients usually occurs in a context of immunosuppression-based therapy, with only a few case reports describing its development in patients presenting no known risk factors. Moreover, the clinical presentation of primary colorectal lymphomas may be difficult to distinguish from an acute flare of ulcerative colitis (UC). <b><i>Case Presentation:</i></b> We present a case of non-Hodgkin lymphoma of the rectum in a 42-year-old male with a 7-year history of UC and no previous exposure to immunomodulatory agents. He presented with a history of mucous diarrhoea, tenesmus, proctalgia and weight loss, refractory to optimized therapy. A lower gastrointestinal endoscopy was performed revealing a circumferential ulcerated lesion of the rectum, from which histopathological analysis established the diagnosis of a non-Hodgkin diffuse large B-cell lymphoma (DLBCL). <b><i>Discussion/Conclusion:</i></b> The present case suggests the existence of alternative mechanisms for the development of DLBCL in UC patients. The clinical presentation mimicking an acute flare of UC posed a diagnostic challenge, highlighting the complexity behind the management of UC patients.