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Leukemia

Leucosis

For medical students2 min readUpdated 2026-10-10

Leukemias are systemic neoplastic diseases of the hematopoietic system with primary involvement of the bone marrow. The pathogenesis is based on malignant transformation of hematopoietic cells leading to genetic instability and impaired maturation.

Primary siteBone marrow

CML chromosomePhiladelphia chromosome (9;22)

Liver weightReaches 5–6 kg

Blast crisisMore than 20% blasts in blood and bone marrow

Nature of the Neoplastic Clone and General Triad

The neoplastic clone in leucosis exhibits genetic instability, a tendency toward phenotypic evolution, and impaired cell differentiation. Over time, a transition from monoclonal to polyclonal proliferation can occur, a striking example of which is the blast crisis stage in chronic myeloid leukemia.

All leukemias share a classic triad of features:

Primary involvement of the bone marrow.
Circulation of neoplastic cells in the peripheral bloodstream.
Early and widespread metastasis (neoplastic infiltration).

Tissue infiltration can be diffuse (causing significant organ enlargement) and focal (formation of nodules invading the capsule).

Clinicopathological Division: Acute and Chronic Forms

Depending on the degree of differentiation of the neoplastic elements, two main groups are distinguished:

Acute leukemias: The substrate consists of immature, low-differentiated blast cells. The disease progresses rapidly, beginning with the malignant transformation of a stem cell, active proliferation of blasts, and replacement of normal bone marrow.
Chronic leukemias: The substrate consists of maturing cytic elements originating from committed precursors. The cells partially retain the ability for terminal differentiation.

The number of neoplastic elements in the blood varies, defining the form of the process: leukemic, subleukemic, leukopenic, or aleukemic.

Chronic Myeloid Leukemia and Genetic Markers

A prominent representative of chronic forms is chronic myeloid leukemia, which is based on an abnormality of the pluripotent stem cell. About 95% of patients have a specific cytogenetic marker — the Philadelphia chromosome (Ph chromosome), which is formed as a result of a reciprocal translocation between chromosomes 9 and 22: `t(9;22) (q34; q11)`.

This results in the formation of the chimeric BCR-ABL gene, which encodes a protein with increased tyrosine kinase activity. This leads to a sharp proliferation of the granulocytic lineage and the replacement of fatty bone marrow with lush, grayish-pink or greenish pyoid tissue.

Pathomorphology of Internal Organs

Systemic infiltration and extramedullary hematopoiesis lead to pronounced changes in parenchymatous organs:

Spleen: Markedly enlarged (mass can exceed 3 kg), firm, with a mottled cut surface. The red pulp is filled with neoplastic elements, follicles are replaced, and infarcts frequently occur due to leukemic thrombi.
Liver: Reaches 5–6 kg, sinusoids within lobules are densely infiltrated by neoplastic cells, and secondary dystrophic changes develop in hepatocytes.
Lymph nodes: In early stages, they enlarge insignificantly, soft, gray-red.

Mnemonic

Triad of syndromes: A-I-D (Anemia, Immunodeficiency, Diathesis/hemorrhagic syndrome). CML stages: Chronic $\rightarrow$ Acceleration $\rightarrow$ Blast crisis (C-A-B).

Frequently asked questions

What types of leukemic manifestations are distinguished depending on the number of neoplastic elements in the blood?

Depending on the content of leukocytes and blast cells in 1 µL of peripheral blood, four forms of leukemias are distinguished.

Leukemic form — tens and hundreds of thousands of cells (total leukocyte count > 30–50 × 10⁹/L), large number of blasts.
Subleukemic form — no more than 15,000–25,000 cells (total leukocyte count above normal, up to 30–50 × 10⁹/L), large number of blasts.
Leukopenic form — total leukocyte count below normal, but blast cells are found in small numbers.
Aleukemic form — total leukocyte count within normal limits, leukemic cells are absent in the blood (found only in hematopoietic tissue).

What are the diagnostic criteria for the blast crisis stage in chronic myeloid leukemia?

Diagnosis of the blast crisis stage in chronic myeloid leukemia is based on laboratory parameters and clinical presentation. The main laboratory criterion is the detection of more than 20% blasts in the bone marrow and peripheral blood. The stage is characterized by polyclonality (appearance of new non-differentiating subclones) and decreased cell maturation. Clinical criteria include:

abrupt worsening of condition;
high resistance to treatment;
rapid enlargement of the spleen (often with organ infarctions);
fever and bone pain (ossalgia);
appearance of dense leukemic infiltrates in the skin;
enlargement of lymph nodes.

How do acute and chronic leukemias differ in their morphological substrate?

Acute leukemias are characterized by a predominance of immature, low-differentiated blast cells. In chronic leukemias, the substrate consists of maturing cytic elements that partially retain the ability to differentiate.

What is the Philadelphia chromosome and in what disease is it found?

It is an abnormal chromosome formed as a result of the t(9;22)(q34;q11) translocation with the formation of the BCR-ABL gene. It is found in approximately 95% of patients with chronic myeloid leukemia.

What are the main causes of the hemorrhagic syndrome in leukemias?

The syndrome is caused by thrombocytopenia due to the suppression of the megakaryocytic lineage, liver damage with impaired synthesis of clotting factors, hypoxia of the vascular wall against the background of anemia, and its direct damage by leukemic cells.

Go deeper

Pathogenesis and cytogenetics of acute myeloid leukemia
Myelodysplastic syndromes and their morphological features
Accelerated phase and blast crisis in chronic myeloid leukemia
Mechanisms of secondary immunodeficiency and necrotizing ulcerative complications
Extramedullary hematopoiesis in leukemias