Reference case: Acute monocytic leukemia

Authors:

Maryam Gbadamosi-Akindele,Janani Baskaran,Sarah Albagoush,Daniel Ermann,Hussain Rangoonwala,Makenzi Stevenson,Brian Hoeynck,Hweisi Tsao,Joleen Fixley.

A 76-year-old man with a previous diagnosis of Chronic myelomonocytic leukemia (CMML) was admitted with hyperleukocytosis (peak white blood cell count of 314.5 x 109 /L). Flow cytometry was significant for monocytes and monocyte precursors that comprised 80% of the white cells. The peripheral blood smear in conjunction with flow cytometry established the diagnosis of the transformation of CMML to Acute monocytic leukemia.

Learning Points:

1) Acute monocytic leukemia is grouped with Acute monoblastic leukemia in the subtype of Acute myeloid leukemia Not otherwise specified (AML NOS) In the recently revised 2016 World Health Organization (WHO) classification of AML.

2) Acute monocytic leukemia and Acute monoblastic leukemia were previously AML-M5b and AML-M5a respectively in the previous French-American -British (FAB) classification system.

3) Acute monocytic leukemia and Acute monoblastic leukemia are differentiated based on the amount of blast cells present. Acute monoblastic leukemia has greater than 80% blast cells while Acute monocytic leukemia has less than 80% blast cells and mainly comprised of promonocytes and monocytes.

4)Promonocytes are often challenging to distinguish from other blast forms on a peripheral blood smear thus placing emphasis on flow cytometry as well as molecular and cytogenetic studies to establish the diagnosis of Acute monocytic leukemia. Distinguishing features of the promonocyte includes the nucleus which is often irregularly shaped with indentations.

5)Hyperleukocytosis is a clinical feature that is associated with Acute monocytic leukemia as witnessed in this patient.

References:

1.Arber DA, Orazi A, Hasserjian R, Thiele J, Borowitz MJ, Le Beau MM, Bloomfield CD, Cazzola M, Vardiman.The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia. Blood. 2016;127(20):2391.

2.Bain B.J What is a promonocyte? Am. J. Hematol. 88:919, 2013.

3.Goasguen JE, Bennett JM, Bain BJ, et al. Morphological evaluation of monocytes and their precursors. Haematologica. 2009;94(7):994-7.

4.Haferlach T, Schoch C, Schnittger S, et al. Distinct genetic patterns can be identified in acute monoblastic and acute monocytic leukaemia (FAB AML M5a and M5b): a study of 124 patients. Br J Haematol 2002; 118:426.

5.National Institute of Health (NIH). The Genetic and Rare Diseases Information Center (GARD) https://rarediseases.info.nih.gov/diseases/525/acute-monoblastic-leukemia. Accessed January 27, 2019.