Flow Cytometry‐Guided Diagnosis of Cortical T‐Lymphoblastic Lymphoma Presenting With a Massive Mediastinal Mass and Borderline Bone Marrow Involvement

BACKGROUND: T-cell lymphoblastic lymphoma (T-LBL) and T-cell acute lymphoblastic leukemia (T-ALL) are considered a single biological entity, distinguished primarily by the extent of bone marrow infiltration. This distinction becomes diagnostically challenging in patients with borderline marrow blast counts and predominantly extramedullary disease, risking overclassification as leukemia. METHODS: We describe a 57-year-old man who presented with dyspnea and a large mediastinal mass causing superior vena cava syndrome, with pleural and pericardial effusions. Diagnostic workup included complete blood count and biochemistry, imaging, fine-needle aspiration, core biopsy, pleural fluid cytology, multiparameter flow cytometry of pleural fluid and bone marrow, immunohistochemistry, and cerebrospinal fluid analysis. RESULTS: CBC and biochemistry were largely unremarkable aside from elevated LDH and CRP. Cytology and biopsy showed a diffuse blastoid lymphoid infiltrate. Pleural fluid flow cytometry identified an aberrant immature T-cell population (cytoplasmic CD3+, CD1a+, CD2+, CD4+, CD5(dim)+, CD7+, CD8+, CD10+, TdT+; surface CD3-, CD34-, MPO-), confirming cortical T-ALL/LBL and excluding B- and myeloid-lineage differentiation. Bone marrow morphology suggested ~10% blasts, but flow cytometry detected only 3.8% CD34+ cells with a myeloid-skewed, non-T-lymphoblastic phenotype, arguing against significant marrow involvement. The patient was classified as T-LBL and treated per the GMALL protocol, achieving complete remission. CONCLUSION: Flow cytometry resolved a discrepancy between morphologic and immunophenotypic assessment of bone marrow involvement, preventing overclassification as T-ALL. Multiparameter flow cytometry is essential for accurate lineage assignment and staging of precursor T-lymphoid neoplasms in cytologic specimens and borderline marrow samples.