Bone marrow histology for the diagnosis of essential thrombocythemia in children: a multi-center Italian study

PURPOSE: To evaluate the relevance of bone marrow histology (BM) in children with a clinical diagnosis of essential thrombocythemia (ET) because little is known about it at present. PATIENTS AND METHODS: We evaluated naïve BM of 21 children with a clinical diagnosis of ET (PedET), of 6 children with secondary thrombocytosis (PedST) and of 36 adults with ET (AdET) diagnosed strictly in agreement with the WHO criteria- Two pathologists blinded to the cause of each child’s thrombocytosis reviewed the slidesy assessing BM cellularity, myeloid to erythroid ratio, myeloid and erythroid maturation, megakaryocyte (MK) number and morphology, presence of loose or tight Mk clusters, density (MKD) and grade of fibrosis. These histological features than was compared with the clinical and bio-molecular characteristics of each child. RESULTS: MK clusters were observed in 21 cases of PedET and BM fibrosis in 6 of them, but not in cases of PedST. A significantly higher number of MK/mm2 was found in PedET (37.5) than in PedST (9.2) (p<0.001). No differences emerged between the cases of PedET and AdET in terms of the percentage of patients with an abnormal MK morphology, MK clusters, or BM fibrosis. BM histology confirmed ET in 19 children, and suggested masked Polycythemia (mPV) in 1 child, and ST in one. CONCLUSION: An accurate diagnosis comprehending BM histology is necessary for the diagnosis of Ph- MPN in children as in adults, BM can identify ET, prefibrotic myelofibrosis and mPV, and distinguish ST from primary thrombocytosis.