Rapid On-Site Cytologic Evaluation-Facilitated Diagnosis of Fibrin-Associated Large B-Cell Lymphoma in a Right Atrial Fibrin Thrombus: A Case Report.

Background: Fibrin-associated large B-cell lymphoma (FA-LBCL) is a rare extranodal lymphoma confined to fibrin deposits within cardiovascular structures and other anatomically restricted spaces. Due to its non-mass-forming nature and nonspecific clinical presentation, preoperative diagnosis remains challenging.

Case presentation: A 47-year-old woman with a history of mitral valve repair and pacemaker implantation presented with a right atrial mass extending from the coronary sinus. A catheter-based biopsy with rapid on-site cytologic evaluation (ROSE) revealed scattered large atypical lymphoid cells. Histology confirmed FA-LBCL, composed of CD20-positive cells with a high Ki-67 index (∼90%) and negative Epstein-Barr virus (EBV)-encoded RNA. The disease was staged as IE, with no evidence of extracardiac involvement. Despite no myocardial invasion, the patient opted for six cycles of R-CHOP and remains disease-free 2 years post-diagnosis.

Conclusion: This case highlights the diagnostic utility of ROSE in FA-LBCL, particularly in guiding sample adequacy and expediting ancillary studies. Given the focal and sparse distribution of neoplastic cells, early recognition through ROSE may prevent misdiagnosis and unnecessary procedures. As EBV-negative FA-LBCL represents a distinct entity with evolving molecular insights, further research is warranted to delineate its pathogenesis and optimal management strategies.