From
http://www.appliedradiology.com/case/case.asp?Id=606&IssueID=154&CatID=43&SubCatID=101&ThreadID=&Quiz=Extramedullary hematopoiesis is the production of blood elements outside the bone marrow cavity...Extramedullary hematopoiesis occurs in severe chronic anemia, which can arise from deficiency disorders such as vitamin B12 or folate deficiency2 or from disorders affecting pluripotent stem cells. The latter are classified as: a) stem cell dysplasia, as in thalassemia, sickle cell hemoglobinopathy, and hereditary spherocytosis...Extramedullary hematopoiesis occurs as a compensatory mechanism for abnormal hematopoiesis when normal red marrow is unable to function because of deficiency disorders or because of various pluripotent stem cell disorders. Most often, it is microscopic and asymptomatic but can sometimes produce organomegaly, tumor-like masses and even life-threatening symptoms. Extramedullary hematopoiesis should be considered in the differential diagnosis in a patient with chronic severe anemia and a paraspinal mass.
Basically, Extramedullary hematopoiesis is the reaction of the body to a low hemoglobin state. Because the bone marrow is not producing sufficient red blood cells, the body tries to compensate by growing red cells elsewhere. This is usually at the cellular level, where it normally is not diagnosed, although it is known to show up on some scans, where it may be misinterpreted as lymphoma because the reaction is identical in the tests. This can potentially lead to treatment for lymphoma that doesn't exist in the patient. Extramedullary hematopoiesis becomes a problem when it grows as a mass in certain locations, like against the lungs, as with Courtenay, or against the spine, as with Nur and pastel_lillian. If the mass grows large enough, it can cause serious injury to the spine. By the way, Courtenay is a thal minor, so doctors everywhere should take note and throw out the books on this. Extramedullary hematopoiesis occurs in thal minor, intermedia and major. The books will tell you that it is found in intermedia but we can't live by books. In our real world, extramedullary hematopoiesis does often occur in majors and I suspect, also in many minors who live with low Hb.
As Sajid and Nur have stated, the first line of treatment is keeping the Hb level up. Nur's problem developed because she has difficulty finding matched blood and often her Hb drops to very low levels. This troubles me greatly, as Nur is a mom (who had her child without any hormones or help, so yes, it can happen), and also one of my dearest friends. Her doctor's strategy is to try and keep her Hb up so the mass doesn't grow any further. If it does, she most likely will require radiation treatments to shrink it. Having talked to people who have had radiation for different cancers, I will say that it isn't as bad as one might imagine, so if it becomes the only choice, please have some confidence because the alternative of doing nothing can actually lead to death.
There are two other ways that extramedullary hematopoiesis is dealt with. One is surgery to remove the mass. This can be dangerous if the mass is pressing on the spine. The other treatment is more experimental and has been mentioned here in this thread and also earlier at
http://www.thalassemiapatientsandfriends.com/index.php?topic=1207.msg12515#msg12515 Hydroxyurea is used to increase Hb which will also suppress the further formation of the extramedullary hematopoiesis. This is somewhat experimental but it may have application outside use with intermedias, just as raising Hb in majors with hydroxyurea may have some application.
You should ask your doctor about hydroxyurea and if it would be appropriate to try in your case. You should also prepare yourself mentally for the prospects of having radiation treatments. If the growth gets too large and endangers your spine, there will be little choice.