Beyond the Surface: Duodenal‐Type Follicular Lymphoma Diagnosed Despite Normal‐Appearing Duodenal Mucosa and Absence of Focal Duodenal FDG Uptake

Duodenal‐type follicular lymphoma (DTFL) is an uncommon and often indolent extranodal lymphoma that may be difficult to recognize when characteristic endoscopic or imaging findings are absent. We report a 60‐year‐old man evaluated for progressive fatigue, epigastric pain, and iron deficiency whose esophagogastroduodenoscopy showed normal‐appearing duodenal mucosa. Random duodenal biopsies revealed follicular lymphoma with morphology and immunophenotype consistent with follicular lymphoma, duodenal type, in the absence of systemic follicular lymphoma. Staging positron emission tomography/computed tomography (PET/CT) showed no focal duodenal fluorodeoxyglucose uptake but demonstrated equivocal low‐level uptake in mildly prominent mesenteric lymph nodes. Given unclear staging and persistent gastrointestinal symptoms affecting quality of life, the patient was treated with rituximab monotherapy. Follow‐up PET/CT findings were consistent with complete metabolic response, and repeat duodenal biopsies showed no residual lymphoma. This case highlights the diagnostic value of random duodenal biopsy in unexplained iron deficiency and emphasizes the staging complexity of DTFL when PET/CT findings are equivocal.