or normal red cell mass in presence of normal iron stores B4. No evidence of chronic myeloid leukemia But the Philadelphia chromosome may be present in up to 10% of cases. Patients with the Philadelphia chromosome have a potential for the development of acute leukemia, especially acute lymphocytic leukemia. B5. No evidence of myelofibrosis no collagen fibrosis and ≤ grade 2 reticulin fibrosis (using 0–4 scale) B6. No evidence of a myelodysplastic syndrome no significant dysplasia no cytogenetic abnormalities suggestive of myelodysplasia Treatment Indications Not all those affected will require treatment at presentation. People are usually split up into low and high risk for bleeding/blood clotting groups (based on their age, their medical history, their blood counts and their lifestyles), low risk individuals are usually treated with aspirin, whereas those at high risk are given hydroxycarbamide and/or other treatments that reduce platelet count (such as interferon-α and anagrelide). Agents Hydroxycarbamide, interferon-α and anagrelide can lower the platelet count. Low-dose aspirin is used to reduce the risk of blood clot formation unless the platelet count is very high, where there is a risk of bleeding from the disease, and hence this measure would be counter-productive as aspirin-use increases the risk of bleeding. The PT1 study compared hydroxyurea plus aspirin to anagrelide plus aspirin as initial therapy for ET. Hydroxyurea treated patients had a lower incidence of arterial thrombosis, lower incidence of severe bleeding and lower incidence of transformation to myelofibrosis, but the risk of venous thrombosis was higher with hydroxycarbamide than with anagrelide. It is unknown whether the results are applicable to all ET patients. In people with symptomatic ET and extremely high platelet counts (exceeding 1 million), plateletpheresis can be used to remove platelets from the blood to reduce the risk of thrombosis. Prognosis Essential thrombocythemia is sometimes described as a slowly progressive disorder with long asymptomatic periods punctuated by thrombotic or hemorrhagic events. However, well-documented medical regimens can reduce and control the number of platelets, which reduces the risk of these thrombotic or hemorrhagic events. The lifespan of a well-controlled ET person is well within the expected range for a person of similar age but without ET. ET is the myeloproliferative neoplasm least likely to progress to acute myeloid leukemia. Epidemiology The incidence of ET is 0.6-2.5/100,000 per year, the median age at onset is 65–70 years and it is more frequent in females than in males. The incidence