Extramedullary hematopoiesis (EMH) is compensatory blood cell production outside bone marrow. Most common causes are myelofibrosis, thalassemia major, sickle cell disease, and chronic hemolytic anemias. Retroperitoneal/paravertebral EMH typically presents as bilateral symmetric paravertebral masses, and this symmetric pattern strongly supports the diagnosis. Liver and spleen usually show accompanying hepatosplenomegaly — the most common EMH locations are spleen, liver, and paravertebral areas. On MRI, T1 intermediate-to-high signal (from fat component within active hematopoietic tissue), T2 variable signal (low signal foci due to hemosiderin deposition). Signal loss on chemical shift imaging confirms intracellular fat and is a critical finding for differentiation from lymphoma. Treatment targets the underlying disease and biopsy is usually not needed.
Age Range
30-70
Peak Age
50
Gender
Equal
Prevalence
Rare
EMH develops as a compensatory mechanism when bone marrow is insufficient or replaced. In myelofibrosis, marrow is replaced by fibrosis and hematopoietic stem cells migrate to spleen, liver, and retroperitoneal/paravertebral areas. In thalassemia, ineffective erythropoiesis expands marrow and triggers EMH — beta-globin chain defect causes early destruction of erythroid precursors, and bone marrow shows compensatory hyperplasia but when insufficient, extramedullary sites are activated. Paravertebral EMH shows extraosseous extension from vertebral marrow through periosteum — hematopoietic stem cells migrate from vertebral marrow through periosteal channels and foramina to the paravertebral space. Active hematopoietic tissue contains fat and erythroid precursors, showing intermediate-to-high T1 signal from fat component. Hemosiderin deposition (from increased iron metabolism as a byproduct of erythropoiesis) creates low T2 foci — superparamagnetic properties disrupt local magnetic field homogeneity. Homogeneous enhancement reflects vascularity of active hematopoietic tissue.
Bilateral symmetric paravertebral masses + absence of bone destruction + intermediate-to-high T1 signal (fat component) + chemical shift opposed-phase signal loss combination is pathognomonic for EMH — confirms diagnosis in myelofibrosis/thalassemia clinical context and usually makes biopsy unnecessary.
On contrast-enhanced CT, bilateral symmetric, homogeneous, soft tissue density paravertebral masses are seen. Moderate homogeneous enhancement is present. No necrosis, calcification, or cystic change expected. Adjacent to vertebral bodies but no bone destruction — this is extremely important for differentiating from metastasis and lymphoma. Masses are typically at lower thoracic and upper lumbar vertebral levels.
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Bilateral symmetric paravertebral masses are seen without bone destruction; extramedullary hematopoiesis should be considered.
On T1, EMH shows intermediate-to-high signal — reflecting the fat and erythroid cell mixture of active hematopoietic tissue. Mild signal loss on fat suppression confirms the fat component. This T1 signal characteristic is the most valuable MR finding for differentiation from lymphoma (low T1) and has near-diagnostic value in the appropriate clinical context.
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Paravertebral masses show intermediate-to-high T1 signal with mild signal loss on fat suppression; consistent with active hematopoietic tissue.
On T2, EMH shows variable signal. Active hematopoietic areas show intermediate T2 signal, hemosiderin deposition areas show low T2 signal (blooming). In chronic EMH, hemosiderin may be dominant with dominant low T2 signal. This hemosiderin deposition is a byproduct of ineffective erythropoiesis — iron metabolism is increased and free iron is stored as hemosiderin. On GRE sequences, hemosiderin shows more prominent signal loss.
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The masses show variable T2 signal with hemosiderin foci and blooming artifact.
On chemical shift (opposed-phase) imaging, EMH shows signal loss — confirming microscopic fat-water mixture. Normal signal on in-phase, significant signal drop on opposed-phase. This finding is of critical value for differentiating from lymphoma because lymphoma does not contain intracellular fat and shows no chemical shift loss. Signal loss ratio should not be confused with India ink artifact — India ink is an interface artifact while EMH signal loss is diffuse throughout the entire mass.
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Significant signal loss on opposed-phase chemical shift imaging confirms microscopic fat-water mixture; consistent with EMH.
On Tc-99m sulfur colloid scintigraphy, EMH foci show increased uptake — proving that active hematopoietic tissue contains reticuloendothelial system cells. This scintigraphic finding confirms the EMH diagnosis and is considered the gold standard for differentiation from other paravertebral masses. Uptake correlates with splenomegaly and hepatomegaly.
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Increased uptake in paravertebral masses on Tc-99m sulfur colloid scintigraphy is consistent with active hematopoietic tissue (EMH).
On DWI, EMH does not show marked diffusion restriction — ADC values are generally in the intermediate-to-high range (1.0-1.8 × 10⁻³ mm²/s). This finding is valuable for differentiation from high-cellularity lymphoma (ADC 0.4-0.8). Active hematopoietic areas may show mild diffusion restriction but not as prominent as in lymphoma.
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No marked diffusion restriction is seen in paravertebral masses with ADC values higher than lymphoma; consistent with EMH.
Criteria
EMH developing on primary or secondary myelofibrosis background, JAK2 V617F mutation frequent
Distinct Features
Massive splenomegaly (spleen >20 cm), leukoerythroblastic blood picture, marrow fibrosis (reticulin/collagen), paravertebral masses may be extensive and large
Criteria
On thalassemia major or intermedia background, presents at young age, chronic transfusion requirement
Distinct Features
Marrow expansion (crew cut appearance — trabecular hypertrophy in calvarium), widespread iron deposition (low T2* — liver, heart, pancreas), young age onset
Criteria
On background of sickle cell disease, hereditary spherocytosis, autoimmune hemolytic anemia, or other chronic hemolytic anemias
Distinct Features
Complications specific to hemolytic anemia accompany — bone infarcts and avascular necrosis in sickle cell disease, gallstones and cholecystectomy history in spherocytosis
Distinguishing Feature
Lymphoma shows low T1 signal and no chemical shift loss; EMH shows high T1 (fat) and positive chemical shift — this difference is the most reliable differential criterion between the two diseases
Distinguishing Feature
Ganglioneuroma usually solitary, homogeneous T2 hyperintense with delayed enhancement; EMH bilateral symmetric, T1 high signal (fat) and clinical hematological anemia context
Distinguishing Feature
Neurofibroma shows target sign and is NF1-associated; EMH is differentiated by fat signal, hemosiderin deposition and hematological disease context
Urgency
routineManagement
medicalBiopsy
Not NeededFollow-up
6-monthEMH treatment targets the underlying disease. Ruxolitinib (JAK2 inhibitor) is primary treatment in myelofibrosis and may reduce splenomegaly and EMH masses. Regular transfusion and chelation therapy (deferasirox, deferoxamine) for thalassemia — prevention of iron overload is critical. Low-dose radiotherapy (10-20 Gy) is effective for spinal cord compression or large symptomatic masses; hydroxyurea may be an alternative by increasing HbF production. Biopsy is usually not needed — clinical context (known hematological disease) + characteristic imaging findings (bilateral symmetric paravertebral masses, T1 high signal, chemical shift loss) + Tc-99m scintigraphy is diagnostically sufficient. Asymptomatic masses can be followed; symptomatic masses (pain, neurological deficit) require treatment.
EMH may shrink with treatment of underlying hematologic disease. Symptomatic masses (such as spinal cord compression) are treated with radiotherapy or surgery. Hydroxyurea may reduce EMH. Biopsy is rarely needed — clinical context and imaging are usually sufficient for diagnosis.