Extramedullary hematopoiesis (review): typical thoracic imaging findings

Introduction

In the adult population, hematopoiesis occurs normally in the marrow of long bones, ribs and vertebrae^(1,2). When the primary sites of hemopoiesis fail, as in myelofibrosis and in hemoglobinopathies, various extramedullary sites take on the role of blood formation. Extramedullary hematopoiesis (EMH) involves more frequently the liver (hepatomegaly), the spleen (splenomegaly), and the paraspinal regions of the thorax^(1-8). In addition to these common sites of EMH, the process can involve any organ or tissue, and it can often manifest as a mass mimicking a neoplasm^(1,2,11-19). Usually, symptoms are caused by the mass effect of the extramedullary hematopoiesis. Knowing the imaging findings compatible with EMH in correlation with biopsy and pathological exam will exclude a neoplasm or a non-neoplastic process, being important for the management and prognosis of the patients. Also, it is very important to know the patient’s history and antecedents^(16,17).

Imaging findings

On chest X ray, the typical findings of extramedullary hematopoiesis consist in bilateral or unilateral, smooth, sharply delineated, often lobulated paraspinal masses without erosion of the vertebral bodies or ribs. Occasionally, they can be seen along the entire length of the vertebral spine (Figure 1).

Calcifications are atypical. Rib expansion and rarely pulmonary infiltrates can be found in EMH^(4-8).

Computed tomography (CT) of the thorax may reveal unilateral or bilateral symmetrical paravertebral masses, vertebral and ribs involvement (Figures 2 and 3). 

Magnetic resonance imaging (MRI) of the spine may relive paravertebral and epidural mass with mass effect and compression of the adjacent thoracic spinal cord (Figure 4). Signal characteristics of extramedullary hemato­poiesis are: intermediate T1 signal intensity or low T1 signal intensity due to massive iron deposition in repeated transfusions. T1 weighted images (wi) with paramagnetic contrast material demonstrate variable enhancement or no enhancement in case of massive iron deposition^{(3,11,12,17-19)}.

Also, in T2 weighted images, when there is excessive iron in the erythroid tissue images, extramedullary hematopoiesis is hypointense relative to the spinal cord⁽¹¹⁾. Hematopoietic tissue is highly vascular but gadolinium enhancement of EMH is variable, showing moderate enhancement, strong enhancement, or no enhancement^(3,11,17).

Discussion

Extramedullary hematopoiesis occurs as a compensa­tory mechanism for abnormal hematopoiesis, due to the proliferation of hematopoietic cells outside of the bone marrow in response to production of too few blood cells to satisfy the body’s demand^(1,2). Extramedullary hematopoiesis occurs in hemoglobinopathies, myeloproliferative disorders, or bone marrow infiltration^(1-19). Mye­lo­proliferative disorders include chronic myelogenous leukemia, polycythemia vera, essential thrombocytosis, and myelofibrosis with myeloid metaplasia. Hemoglobinopathies include sickle cell disease and thalassemia⁽³⁾.

EMH mainly involves the reticuloendothelial system (liver, spleen, and lymph nodes), but it is also known to occur in every organ of the body, including the thyroid, prostate, pericardium, kidney, and lungs^(1,16-19). Rarely, it can cause cord compression, pleural effusion, massive hemothorax and respiratory failure^{(3-6,12,16,17)}. Extramedullary hematopoiesis should be considered in the differential diagnosis in a patient with chronic severe anemia and a paraspinal mass^(6,7,12).

Paraspinal masses are the most common site of EMH after hepatosplenomegaly and the most common site to present, typically discovered incidentally on a thoracic radiography or using CT^(16,17).

Chest radiography reveals a sharply outlined, bilateral lobulated paraspinal and rib masses (Figure 1).

Thoracic CT scan may show well-defined unilateral or bilateral paraspinal symmetrical masses, ribs expansion, without or with intralesional fine linear calcifications (Figures 2 and 3). Especially the posterior parts of the ribs may show increased trabeculation and dimensions (Figure 3). Conventional spin echo T1 MRI demonstrate that paravertebral masses and the epidural mass have the same signal characteristics with the adjacent vertebral body marrow (Figure 4). The epidural mass may compress the adjacent spinal cord^(12,16).

Patients with hemoglobinopathies are more likely to have EMH in paraosseous locations, whereas patients with myeloproliferative disorders are more likely to have extraosseous masses⁽¹¹⁾. More commonly observed areas of EMH include the paraspinal regions of the thorax⁽¹²⁾, liver and spleen, but it has been reported in other locations, including the adrenal gland, stomach, bowel, dura mater, and breast^(11,13-15). Only a few cases of perirenal and pelvicalyceal EMH have been reported in the literature^(11,15,16).

Intrathoracic EMH is a rare cause of paraspinal mass and should be differentiated from other costovertebral angle masses, such as neurogenic tumors, lymphoma, metastasis and paravertebral abscess^(1,8,12). EMH deposits have a high tendency to hemorrhage, given their vascular nature, therefore careful planning prior to any biopsy attempt is important⁽¹⁶⁾. Fine-needle aspiration with pathological exam can be used to make the final diagnostic^(16,19). It has been suggested that fine-needle aspiration is preferred to open-lung biopsy for suspected pulmonary EMH involvement, given the risk of hemorrhagic complications^(18,16,19).

Extramedullary hematopoiesis usually regresses or disappears after the treatment with blood transfusion with the purpose to limit hematopoietic stimulus, splenectomy in cases of spherocytosis, or radiation therapy^(1,9,10,16). The use of low-dose radiation therapy has been reported for paraspinal lesions affecting the spinal cord or peripheral nerves^(16,18).

Conclusions

Based on radio-imaging characteristics and the clinical findings, it is important to recognize and diagnose thoracic EMH and to differentiate extramedullary hematopoiesis from other mediastinal masses, such as multiple ganglio­neuroma, myeloma, lymphoma, metastasis or multiple abscesses. Fine-needle aspiration with pathological exam can be used to establish the final diagnosis.   

Corresponding author: Ioana G. Lupescu E-mail: ilupescu@gmail.com

Conflict of interest: none declared.

Financial support: none declared.

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