Risk of Thromboembolic Events in ET and Early/Prefibrotic PMF: The Relevance of an Accurate Histological Diagnosis

Essential Thrombocytemia (ET) is a myeloproliferative neoplasm characterized by increased risk of vascular events. Established thrombosis risk factors are age and previous vascular events. The clinical and prognostic relevance of WHO histologic criteria for ET and prefibrotic/early Primary Myelofibrosis (PMF) has been well recognized. Our aim was to evaluate the correlation between histologic interpretation and vascular events in our series of thrombocytemias.

From our files, we retrieved all patients consecutively diagnosed as having ET with complete clinical data (N = 283) who had undergone to a bone marrow trephine biopsy before any treatment at or within 1 year of diagnosis (N= 133). The histologic slides were reviewed in order to separate true ET cases from early/prefibrotic PMF; vaso-occlusive events at diagnosis and in the follow-up were than compared in the two groups.

Histologic review reclassified 61 cases as ET and 72 cases as prefibrotic/early PMF. Prefibrotic/early PMF showed a significant higher prevalence of thrombosis history and thrombotic events at diagnosis, and an increased leukocyte count than ET (22% vs 8%, 15.2% vs 1.6%, 8389/mmc vs 7500/mmc, respectively); furthermore, venous thromboses (mainly atypical) were relatively common in PMF, as opposed to WHO-defined ET. During follow-up, patients with prefibrotic PMF, although younger, showed a significant higher risk of developing thrombosis: the 15-year risk of thrombosis was 48% in prefibrotic PMF (grade 0), 16% in early PMF (grade 1, 2) and 17% in ET. Multivariate analysis confirmed that age and histopathology are independent risk factors for thrombosis during follow-up. Patients older than 60 or with prefibrotic PMF are high risk patients whereas those younger and with non prefibrotic PMF or ET should be considered at low risk (20-year risk of thrombosis 47% vs 4%, p=0.005).

The results of present study indicate prefibrotic PMF as a myloproliferative neoplasm with the highest tendency to develop vascular events compared to early PMF and ET. Therefore we suggest to include histopathology interpretation in the risk stratification of so-called ET patients.

No relevant conflicts of interest to declare.