Supplementary Material for: Transformation of primary small intestinal follicular lymphoma to CD20-negative diffuse large B-cell lymphoma following rituximab treatment: A case report

We report a case of a 64-year-old woman initially diagnosed with primary small intestinal follicular lymphoma who subsequently developed a transformation to diffuse large B-cell lymphoma (DLBCL) two years after rituximab treatment. Double-balloon enteroscopy revealed progression in mucosal lesions and raised suspicion of transformation. However, the endoscopic biopsy was inconclusive due to the loss of CD20 expression, likely resulting from prior rituximab therapy. Surgical resection was required to relieve obstruction and establish a definitive diagnosis. Histological eamination confirmed transformation to CD20-negative DLBCL. This case highlights the diagnostic limitation of endoscopic biopsy following rituximab therapy and underscores the importance of surgical intervention in cases with suspected histologic transformation and obstructive symptoms.