Blastoid Mantle Cell Lymphoma in the Leukemic Phase: Resolving a Morphological Dilemma Through Flow Cytometry.

Leukemic conversion of nodal mantle cell lymphoma (MCL) is most often detected as incidental lymphocytosis on routine blood counts, often mimicking chronic lymphocytic leukemia, with flow cytometric immunophenotyping being the gold-standard test for diagnosis. Blastoid MCL closely mimics acute lymphoblastic leukemia (ALL), thus posing significant diagnostic challenges. Blastoid transformation of MCL is usually associated with TP53 mutations, homozygous deletions of CDKN2A and CDKN2C, amplifications and overexpression of CDK4, and occasionally microdeletions of RB1, and is associated with an aggressive disease course. Equivocal cases require histopathological evaluation, fluorescent in-situ hybridization, or molecular studies for confirmation. We report the case of an 82-year-old female who presented with lower respiratory tract infection and was found to have severe anemia (hemoglobin: 45 g/L) and marked leukocytosis (325 × 10⁹/L) with 63% blastoid-appearing cells on peripheral blood smear, initially suggestive of acute leukemia. Flow cytometric immunophenotyping demonstrated bright CD45 expression with low side scatter and positivity for CD19, CD20, CD38, CD5, CD79b, and FMC7 with negativity for CD34, CD23, CD200, and CD10, suggesting blastoid transformation of MCL rather than de novo ALL. This case highlights the critical role of flow cytometry in distinguishing blastoid MCL from acute leukemia, thereby preventing misdiagnosis and ensuring appropriate therapeutic decision-making.