We thank Juergen Thiele and Hans Kvasnicka for commenting on our recently published paper including 605 patients with essential thrombocythemia (ET).1 We provided evidence that progression to myelofibrosis (post-ET MF) has a prevalence of 2.8% (10-year risk of 3.9%), and that progression to acute leukemia (AL) has a prevalence of 2.3% (10-year risk of 2.6%). The first question of Thiele and Kvasnicka concerns the evolution of ET in post-ET MF and AL. They asked that the prevalence of post-ET MF and AL in patients diagnosed according to the PVSG criteria2 and with the WHO criteria,3 be evaluated separately. The analysis suggested would be strongly biased by the fact that PVSG-classified patients have longer follow-up than WHO-classified patients. In the paper, we mentioned that the longer the follow-up, the higher the risk of transformation into myelofibrosis or leukemia. We regret that their request could not be satisfied, but a direct comparison of these two cohorts with different follow-up may give a misleading message. The second question from Thiele and Kvasnicka concerns the diagnostic differentiation between ET and prefibrotic/early fibrotic phase of primary myelofibrosis (PMF), an entity recognized on the basis of bone marrow features by the WHO classification of 2001.4 However, the recent WHO classification requires the combination of histological picture, clonal markers and clinical parameters to diagnose PMF at prefibrotic or fibrotic phase.3 In our series we excluded cases of PMF (excluded combination of leukoerythroblastosis, anemia, elevated LDH, spleen enlargement). Concerning the discussion on sequential bone marrow evaluations, we perform bone marrow biopsy at diagnosis in all the patients and during follow-up when we suspect clinical progression of the disease. We are glad to know that the prevalence of myelofibrosis reported by Thiele and Kvasnicka ranges between 2.8% and 3.5%. We find that this is a reassuringly low prevalence for patients with ET.
References
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