= abnormal deposits of hematopoietic tissue outside bone marrow ← deficient bone marrow blood cell production

Etiology: prolonged erythrocyte deficiency due to

1. destruction of RBCs:
   • congenital hemolytic anemia (sickle cell anemia, thalassemia, hereditary spherocytosis), acquired hemolytic anemia, idiopathic severe anemia, erythroblastosis fetalis
2. inability of normal blood-forming organs to produce RBCs: iron deficiency anemia, pernicious anemia, myelofibrosis, myelosclerosis, polycythemia vera, carcinomatous / leukemic / lymphomatous replacement depletion of bone marrow (chronic myelogenous leukemia, Hodgkin disease)

◊NO hematologic disease in 25%

Histo: erythroid precursors in extramedullary sites

• absence of pain, bone erosion, calcification; chronic anemia

Sites: in areas of fetal erythropoiesis

• Spleen
  • splenomegaly
  • focal isodense masses on enhanced CT
• Liver, lymph nodes
• Thorax: mediastinum, heart, thymus, pleura, lung
  • uni- / bilateral smooth lobulated paraspinal masses between T8 and T12
  • anterior rib ends expanded by masses
• Spine
  • Most commonly afflicted in thalassemia
  • back pain, symptoms of spinal cord compression
  • coarsened trabeculation
  • extramedullary hematopoiesis in epidural space
• Adrenal glands
• Renal pelvis
• Retroperitoneum (uncommon)
  Site: perirenal (uncommon)
  • hyper- / isoattenuating masses in paravertebral region ± macroscopic fat on CT
  • hypointense mass on T1WI + T2WI ← red marrow / hemosiderin content
  • hyperintense on T1WI + T2WI ← fat tissue
  • variable mild enhancement
• Gastrointestinal lymphatics
• Dura mater (falx cerebri and over brain convexity)
  • expanded diploic space
• Cartilage, broad ligaments
• Thrombi, adipose tissue
• Bone marrow reconversion = conversion of fatty to hematopoietic marrow
  Sequence: vertebrae >flat bones of pelvis >long bones of extremities (proximal metaphysis >distal metaphysis >diaphysis)
• lack of calcification / bone erosion
• signs of hemochromatosis