Fact-sheet: Extramedullary hematopoiesis


Updated on 08/17/2020 at 3:34 PM

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Definition

Extramedullary hematopoiesis is a non-neoplastic proliferation of hematopoietic tissue outside the bone marrow, occurring reactively in response to various hematologic disorders.

The main etiologies are constitutional or acquired hemolytic anemias, chronic anemias, causes of loss of control of hematopoietic stem cell differentiation, and destruction of blood cell line precursors due to marrow invasion by carcinomatous cells.

The main sites of involvement are the embryonic sites of hematopoietic tissue proliferation (liver, spleen), as well as lymph nodes, the posterior mediastinum, the kidneys, and the thoracic spine.

Clinical features

Few clinical manifestations, except in cases of epidural involvement:

  • Signs of spinal cord or nerve root compression.
  • Cough due to pleural irritation.

CT

Non-specific appearance of intrahepatic or intrasplenic foci of extramedullary hematopoiesis:

  • spontaneously hypodense relative to the hepatic parenchyma
  • well-circumscribed nodular lesions, non-calcified, may contain fat,
  • variable, intense enhancement during the portal venous phase.

MRI

Classically, a well-defined, subpleural, round to oval, symmetric paravertebral mass is found, without adjacent bone lysis.
Reflects marrow infiltration.
The appearance depends on the proportion of active red marrow versus inactive yellow marrow:

active regenerating focus: intermediate T1, hyperintense T2, minimal enhancement after injection

Inactive focus: hypointense T1, hypointense T2, no enhancement.

Fat hyperintensity on T1- and T2-weighted sequences

Nuclear medicine

Technetium-99m scintigraphy (particularly for the diagnosis of intrathoracic foci)

Management

Benign, no treatment required.
Treatment of the underlying cause.

No impact on therapeutic management.

Biopsy and pathologic analysis in cases of diagnostic uncertainty.

Differential diagnosis

In the thoracic region: neurogenic tumor, lymphoma, mesenchymal tumor