Table of Contents
Definition / general | Case reports | Microscopic (histologic) description | Microscopic (histologic) images | Peripheral smear images | Positive stains | Negative stains | Flow cytometry description | Electron microscopy images | Molecular / cytogenetics description | Differential diagnosisCite this page: Mihova, D. M5b. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/leukemiaacutemonocyticleukemiam5b.html. Accessed April 24th, 2024.
Definition / general
- Acute monocytic leukemia (M5b)
- 3 - 6% of AML
- Affects all ages
- Mature monocytes or promonocytes predominate in peripheral blood (< 80% of monocyte lineage cells are monoblasts, usually < 20%)
- Treatment may cause tumor lysis syndrome, DIC and falsely elevated platelet counts (Arch Pathol Lab Med 1999;123:1111)
Case reports
- 47 year old man with spontaneous remission after infection (Int J Lab Hematol 2007;29:386)
- 50 year old man with cutaneous disease (Arch Pathol Lab Med 2005;129:425)
- 63 year old man with preceding aleukemic leukemia cutis (Case Rep Oncol 2011;4:547)
- 74 year old man with mycosis fungoides now with increased circulating immature mononuclear cells (Univ Pittsburgh case #460)
- 77 year old man with coexisting myeloma (Arch Pathol Lab Med 2003;127:1506)
- With mononucleosis syndrome due to varicella zoster virus (Eur J Haematol 2002;68:236)
Microscopic (histologic) description
- Leukemic cells are often promonocytes with less basophilic cytoplasm and more azurophilic granules than monoblasts
- Have folded or cerebriform nuclei with fine chromatin
- Erythrophagocytosis is common
Microscopic (histologic) images
AFIP images
Monocytic cells have range of differentiation
2 myeloblasts and 3 promonocytes
Moderate granular cytoplasm
Nonspecific esterase positive
Peripheral smear images
AFIP images
Promonocytes have abundant cytoplasm
Negative stains
Flow cytometry description
Electron microscopy images
AFIP images
Promonocyte has cytoplasm with numerous small cisterna
Molecular / cytogenetics description
- 30% have cytogenetics abnormalities, including 11q23 in 12% (these cases should be classified as a recurrent genetic abnormality)
- FLT3 mutations in 30%
- t(8;16)(p11;p13) fuses MOZ gene at 8p11 with CBP gene at 16p13 and is associated with erythrophagocytosis and coagulopathy
Differential diagnosis
- AML M4
- Malignant histiocytic disorders
- Microgranular acute promyelocytic leukemia
- Myelodysplastic syndrome: to distinguish, count promonoblasts in M5 with monoblasts
