Mosunetuzumab for relapsed follicular lymphoma presenting with chylothorax: A case report and literature review

Follicular lymphoma (FL) is an indolent B-cell non-Hodgkin lymphoma characterized by a relapsing clinical course. Recently, CD20×CD3 bispecific antibodies, including mosunetuzumab, have emerged as effective therapeutic options for relapsed or refractory FL. However, their efficacy in malignant effusions, including pleural effusion, remains unclear. We report a case of relapsed FL complicated by chylothorax that showed a rapid response to mosunetuzumab. A 74-year-old woman with a history of FL, initially treated with R-CHOP and later with rituximab monotherapy, presented with progressive dyspnea and was found to have bilateral pleural effusion. Analysis of pleural fluid revealed chylothorax with infiltration of CD20-positive lymphoma cells, confirmed by flow cytometry and cell-block immunohistochemistry. Following initiation of mosunetuzumab, pleural effusion decreased promptly, with a marked reduction in lymphoma cells in the pleural fluid observed as early as day 2. The patient achieved a complete metabolic response on positron emission tomography-computed tomography (PET-CT) after eight cycles of therapy. This case suggests that mosunetuzumab may exert rapid antitumor activity in both systemic disease and malignant effusions, possibly through distribution into pleural fluid and subsequent T-cell-mediated cytotoxicity. Further studies are warranted to clarify the role of bispecific antibodies in malignant effusions associated with lymphoma.