Myeloproliferative Neoplasms in Patients below 25 Years Old at Diagnosis: A Retrospective International Cooperative Work

Myeloproliferative neoplasms (MPN) are most common in old people (>60 years) and are rarely identified in children and young adults where information about complication rates and long-term data are lacking. To improve our knowledge, we retrospectively collected cases of young patients diagnosed with MPN before 25 years of age and analysed data of their disease to date, including vascular events and disease evolution. Data were collected in 29 hospital centres from 12 countries. Between 1971 and 2018, 335 young patients were diagnosed for an MPN before the age of 25. They were mostly females (n=246; 73.4%) with a median age of 20.3 years at diagnosis (2.5 months-25 years). Essential thrombocythemia was diagnosed in 234 patients (69.8%), polycythemia vera in 60 (17.9%) and myelofibrosis or unclassified MPN in 41 (12.3%). Most of the diagnoses were made following a coincidental blood count analysis (n=75; 51%) some based on symptoms (n=57; 38.8%) or thrombotic events (n=15; 10.2%). In terms of complications before or at diagnosis, 31 (9.3%) patients experienced thrombosis, mostly venous (75%) and 13 (3.9%) had hemorrhage. At diagnosis, the median leukocyte count was 9x109/l (range: 3-22.8), median hemoglobin count 140 g/l (65-220) and median platelet count 900x109/l (99-3290). To assess the diagnosis, 158 patients (47.2%) had had bone marrow aspirates and 214 (63.9%) a bone marrow biopsy. Mutational status was available in 319 (90%) cases: 194 (60.8%) were JAK2V617F positive, 48 (15%) had a calreticulin mutation, 76 (23.8%) were triple-negative and 1 patient had MPL mutation. The median follow-up of the cohort was 7.7 years (0-46.8) with 134 patients (40%) having follow-up for more than 10 years. 81 female patients (32.9%) experienced pregnancies. During this period, 295 patients (88%) received at least one drug for their MPN: 254 patients (77.2%) received antithrombotic drug and 222 patients (66.5%) a cytoreductive drug. As first line of treatment, hydroxycarbamide was given to 111 patients (50%) whereas anagrelide was given to 56 patients (25.2%) and interferon to 50 (22.5%). During the follow-up, 97 patients (29%) experienced at least one complication. In terms of cardiovascular events, 38 (11.3%) patients experienced thromboses with 50 events in total (recurrences in 12 cases), including 33 venous events (66%) of which 15 were localized in the splanchnic territory (45.5%). Hemorrhagic events were recorded in 34 cases (10.1%). During the follow-up, 39 pati