Bone marrow neoplastic

Bone marrow - neoplastic myeloid

AML not otherwise specified

M5a



Last author update: 1 February 2013
Last staff update: 16 February 2022

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PubMed Search: Acute monoblastic leukemia [title] M5a

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Cite this page: Mihova, D. M5a. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/leukemiaacutemonocyticleukemiam5a.html. Accessed August 17th, 2022.
Definition / general
  • Acute monoblastic leukemia (M5a)
  • 5 - 8% of AML
  • Children and young adults
Diagnosis
  • 80%+ of monocyte lineage cells are monoblasts
Case reports
  • 66 year old man with erythropoietin dependent transformation of refractory anemia with ringed sideroblasts into acute monoblastic leukemia (Blood 2001;98:3492)
  • 73 year old woman with coexisting mantle cell lymphoma (Leuk Lymphoma 2005;46:1813)
  • 82 year old man with acute monoblastic leukemia following granular lymphocyte proliferative disorder (Rinsho Ketsueki 2011;52:1870)
Microscopic (histologic) description
  • Hypercellular marrow with large number of monoblasts
  • Monoblasts are large with moderately abundant intensely basophilic cytoplasm, variably basophilic and delicate azurophilic granules but no / rare Auer rods
  • May have pseudopods or vacuoles
  • Have round nuclei and lacy chromatin with one or more prominent nucleoli but no folds
  • Promonocytes have abundant less basophilic cytoplasm with obvious azurophilic granules and nuclei have delicate folds
Microscopic (histologic) images

AFIP images
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Abundant cytoplasm with azurophilic granules

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Monoblasts are large with abundant cytoplasm

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Some monoblasts also show pseudopods

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Monoblasts have variable cytoplasm

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Monoblasts and promonocytes

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Marrow completely replaced by monoblasts


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Monoblasts are large with abundant pale cytoplasm

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Large monoblasts with abundant cytoplasm

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Nonspecific esterase positive

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CD68 #1 (KP-1) positive

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PAS positive

Positive stains
Negative stains
Molecular / cytogenetics description
  • 75% have cytogenetics abnormalities, including 11q23 in 30% (these cases should be classified as a recurrent genetic abnormality)
  • FLT3 mutations in 7%
Electron microscopy images

AFIP images
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Scattered electron dense deposits

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Cytoplasm contains focal area of glycogen deposition

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