Transformation of Primary Small Intestinal Follicular Lymphoma to CD20-Negative Diffuse Large B-Cell Lymphoma following Rituximab Treatment: A Case Report

Introduction: Follicular lymphoma is an indolent B-cell lymphoma that can involve the gastrointestinal tract, most commonly the small intestine. Although rituximab-based therapy is effective, transformation to diffuse large B-cell lymphoma (DLBCL) can occur and becomes difficult to diagnose after CD20 loss. Case Presentation: We report the case of a 64-year-old woman initially diagnosed with primary small intestinal follicular lymphoma who subsequently developed transformation to DLBCL 2 years after rituximab therapy. Double-balloon enteroscopy revealed progression of mucosal lesions, raising suspicion of histologic transformation. However, endoscopic biopsy was inconclusive because of the loss of CD20 expression, likely resulting from prior rituximab therapy. Surgical resection was performed to relieve intestinal obstruction and to establish a definitive diagnosis. Histopathological examination confirmed transformation to CD20-negative DLBCL. Conclusion: This case highlights the diagnostic limitation of endoscopic biopsy following rituximab therapy. Therefore, clinicians should be cautious in relying solely on endoscopic findings and remain open to surgical intervention to achieve a timely and accurate diagnosis.