No Increased Incidence of Bleeding in Essential Thrombocythemia Patients with Extreme Thrombocytosis

CONCLUSIONS About 25% of essential thrombocythemia (ET) patients present with extreme thrombocytosis (ExT), defined as having a platelet count ≥1000 x 10^9/L. ExT patients may have an increased bleeding risk, at least in part associated with acquired von Willebrand disease. There are no specific guidelines for managing ExT patients, and many are started on cytoreduction, even if they are younger and otherwise at low thrombotic risk by the revised International Prognostic Score of Thrombosis for ET (R-IPSET). We analyzed the risk of bleeding and thrombosis in ExT vs. non-ExT ET patients to inform treatment decisions. METHODS : We retrospectively analyzed patients who received targeted gene sequencing at Massachusetts General Brigham / Dana-Farber Cancer Institute from 2014-2021 and met WHO 2016 criteria for ET. We performed medical record review and recorded information on treatment, diagnosis, blood counts, and von Willebrand antigen and activity levels. We abstracted the first major bleed, clinically relevant non-major bleed (CRNMB), and thrombotic event in a patient's disease course. Bleeding was categorized using the International Society on Thrombosis & Haemostasis definitions.We determined the cumulative incidence rates of bleeding and thrombosis in ExT and non-ExT patients with death as a competing event and compared these rates using a Gray test. Risk factors for bleeding and thrombosis were evaluated using univariate and multivariate Fine Gray models. We identified 408 patients diagnosed with ET, of which 344 had a platelet count reported at diagnosis. There were 96 ExT patients and 248 non-ExT patients. Median follow-up was 11 and 6 years, and median platelet count at ET diagnosis was 1194 and 651, respectively. ExT patients, compared with non-ExT patients, were more likely to be younger (51 vs. 58 years old, p<.001), have the CALR mutation (49% vs. 23%, p<.001), have lower IPSET-thrombosis risk (65% vs. 49% very low or low risk, p<.001), and have lower von Willebrand activity levels at diagnosis or in disease course (60% vs. 94%, p<.001, Table 1). There were 7 (7%) major bleeds and 7 (7%) CRNMBs in the 96 ExT patients, and 11 (4%) major bleeds and 22 (9%) CRNMBs in the 248 non-ExT patients (p=.29 for major bleeding, p=.83 for CRNMB). 10 year cumulative incidence of major bleed and CRNMB was not different in ExT vs. non-ExT patients (4% vs. 5%, p=.68 for major bleed; 3% vs. 10%, p=.27 for CRNMB, Figure 1). Despite 65% of ExT patients in our sample categorized as very low or low risk for thrombosis, these patients were more likely to be treated with cytoreduction compared to non-ExT patients (63% vs. 50%, p=.21 for very low risk; 81% vs. 51%, p=.022 for low risk). However, there was no difference in the 10 year cumulative incidence of major bleeding and CRNMB in ExT patients who were treated vs. not treated with cytoreduction (6% vs. 0%, p=.13 for major bleed; 2% vs. 8%, p=.13 for CRNMB). There was also no difference in the cumulative incidence of bleeding in ExT and non-ExT patients who were not treated with cytoreduction (untreated or aspirin alone, 0% vs. 4%, p=.34 for major bleed; 8% vs. 9%, p=.75 for CRNMB). In univariate analysis, diabetes mellitus and thrombosis during disease course were significantly associated with increased combined major bleeding and CRNMB. In a multivariable analysis adjusting for these factors in addition to platelet count ≥1000 x 10^9/L and age at ET diagnosis, first von Willebrand factor activity, aspirin and cytoreduction during disease course, and JAK2 mutation status, diabetes mellitus (HR 3.45, 95% CI 1.31-8.72) and thrombosis during disease course (HR 2.92, 95% CI 1.41-6.03) remained as significant risk factors for bleeding. Cytoreduction was not significantly associated with a lower rate of bleeding in multivariable analysis (HR .42, 95% CI .09-3.06). 10 year cumulative incidence of thrombosis was not different in ExT vs. non-ExT patients (22% vs. 26%, p=.94). We found no difference in the cumulative incidence of bleeding and thrombosis in ET patients with ExT and importantly, no clear role for initiation of cytoreductive therapy in these patients who are otherwise low risk by R-IPSET. This confirms the current recommendations to initiate cytoreduction based on thrombotic risk and suggests that ExT alone should not be an indicator for cytoreduction.