Extramedullary hematopoiesis

Extramedullary hematopoiesis is a response to the failure of erythropoiesis in the bone marrow.

This article aims to a general approach on the condition, for a dedicated discussion for a particularly involved organ, please refer to the specific articles on: 

• extramedullary hematopoiesis in the spleen

• extramedullary hematopoiesis in the adrenal gland

Extramedullary hematopoiesis usually affects visceral organs like the liver, spleen, lymph nodes, and thorax. Less commonly it can affect the pleura, lungs, gastrointestinal tract, breast, skin, brain, kidneys, epidural space, and adrenal glands.

Pathology

Etiology

• myeloproliferative disorders

  • chronic myeloid leukemia

  • polycythemia vera

  • essential thrombocythemia

  • myelofibrosis with myeloid metaplasia

  • Hodgkin lymphoma ⁶

• hemoglobinopathies

  • sickle cell disease

  • thalassemia

  • hereditary spherocytosis ⁵

Radiological features

• most common: diffuse visceromegaly (splenomegaly and hepatomegaly)

  • best evaluated with ultrasound, CT, or MRI

  • lesions are typically hypermetabolic, hence FDG-18 PET avid ⁴

• rarely, can result in focal masses in liver and spleen that need to be differentiated from malignancy

• most common intrathoracic finding is a posterior mediastinal mass

  • may be either unilateral or bilateral

  • smooth, sharply-delineated, often lobulated margins

  • fat can be seen if chronic

  • calcification is very atypical

• other than this, within the thorax, there can be rib expansion and rarely pulmonary infiltrates ⁴

• perirenal soft tissue with normal renal contour can be seen (mimicking lymphoma or Erdheim-Chester disease-like appearance) ⁴; it has been found to be the most common retroperitoneal finding ⁴

• focal or diffuse peritoneal nodules can be seen ⁴

• can present as pre-sacral soft tissue mass ⁴

• epidural soft tissue masses with peripheral fat can be seen in spinal cord or CNS with compression of spinal cord ⁴

CT

• lobulated mass with soft tissue attenuation ⁷

• areas of fat attenuation, if macroscopic fat is present ⁷

• attenuation of lesion is more than that of a skeletal muscle, in case of repeated blood transfusions ⁷

MRI

Signal characteristics of extramedullary hematopoiesis are ⁷:

• T1

  • intermediate signal intensity

  • low signal intensity due to massive iron deposition in repeated transfusions

• T1 C+ (Gd)

  • variable enhancement 

  • no enhancement in case of massive iron deposition

• T2

  • intermediate to high signal intensity (relative to skeletal muscle)

  • low signal intensity due to massive iron deposition in repeated transfusions

Treatment and prognosis

These masses are generally hypervascular with a high chance of bleeding as a complication of biopsy. Biopsy near vital structures like spinal cord is therefore avoided to prevent a risk of spinal cord compression. Fine needle aspiration (FNA) biopsy is a better option at such sites ⁴. 

Treatment includes radiotherapy to the involved site, excision of the mass, or multiple blood transfusions to decrease extramedullary hematopoiesis ⁴.