Bone marrow neoplastic

Bone marrow - neoplastic myeloid

AML not otherwise specified


Topic Completed: 1 February 2013

Minor changes: 11 May 2021

Copyright: 2001-2021,, Inc.

PubMed Search: Acute monocytic leukemia (M5b)

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Cite this page: Mihova, D. M5b. website. Accessed December 3rd, 2021.
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
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

Images hosted on PathOut server:

Blood smears (Wright-Giemsa):
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Promonocytes have abundant cytoplasm with
azurophilic granules that are myeloperoxidase negative,
nuclei have delicate folds, nucleoli are inconspicuous

Bone marrow smears (Wright-Giemsa):
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Monocytic cells have range of
differentiation, promonocytes
have nuclei with delicate folds

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Two myeloblasts (round / oval nuclei, high N/C ratio)
and three promonocytes (abundant cytoplasm
with delicate azurophilic granules,
lobulated nuclei with delicate folds)

Bone marrow biopsy:
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Cells have moderate granular
cytoplasm, nuclei are lobulated and
indented with indistinct nucleoli

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Monocytes and one neutrophil are
intensely positive for nonspecific esterase

Electron microscopy images

Images hosted on PathOut server:
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Promonocyte has cytoplasm with numerous small
cisternae of rough endoplasmic reticulum and
a few scattered dense granules, nucleus is
markedly lobulated with marginated chromatin

Positive stains
Negative stains
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
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