We read with great interest the case study interpretation section (CSI) in the last issue of Cytometry Part B. We perform flow cytometry tests and it is always constructive to challenge our analysis and interpretation skills.
Case 2 of the CSI, reported by Weina Chen , describes a case of an acute leukemia diagnosed as mixed phenotype leukemia T/myeloid. According to the World Health Organization (WHO), myeloperoxidase (MPO) or at least two markers of monocytic differentiation, are required to assign the lineage of a blast population as myeloid, although it does not establishes a minimum percent of myeloid population for the leukemia to be considered of mixed phenotype . In the immunophenotypic analysis of the case only ∼3% of the blasts expressed cytoplasmatic MPO (cMPO). This small subset of blasts could represent normal myeloid progenitor cells since this population was in the compartment where it would be expected to have nonmalignant myeloid blasts , and there was no evidence of cytoplasmatic CD3 coexpression in the FCS files. Also, in the lymph node biopsy, the cells largely lacked MPO and it could have been expressed in myeloid cells instead of blasts.
As suggested recently, a threshold of 10% for cMPO staining detected by flow cytometry in blast cells would be a secure lower limit for the expression of this marker and subsequent classification of the leukemia as myeloid lineage . We believe that a 10% positivity for flow cytometric analysis would be a good choice for the classification of the leukemia as mixed phenotype as well . Although mixed phenotype acute leukemias are rare, more studies are needed in order to make possible the definition of a minimum percentage of expression for lineage determination. These objective evidences are important to define more precise criteria in the next edition of WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues.
Natália Aydos Marcondes,1,2*
Flavo Beno Fernandes,1
Gustavo Adolpho Moreira Faulhaber1,2
1Laboratório Zanol,Porto Alegre, Brazil
2Programa de Pós Graduação em Medicina: Ciências Médicas, Universidade Federal do Rio Grande do Sul, Porto Alegre, Brazil
*Correspondence to: N.A. Marcondes, E-mail: firstname.lastname@example.org