Introduction: Essential thrombocythemia (ET) is a BCR-ABL1 negative myeloproliferative neoplasm with an incidence of 1-2.5/100,000 population per year. It is caused by mutations in the JAK2, CALR, or MPL genes. ET is considered benign, with a low risk of leukemic transformation, a good prognosis, and long-term survival. However, severe complications such as myelofibrosis (MF) and acute myeloid leukemia (AML) can occur. Recent studies indicate ET progresses to MF in 4.8% of cases and to AML in 3.3% of cases. Additionally, leukemia cutis occurs in 3% of AML patients. Here, we report an extremely rare case of ET that progressed to MF and subsequently transformed into leukemia cutis.

Case presentation: We present a unique case report highlighting the clinical progression of a 75-year-old female diagnosed with JAK2-positive Essential Thrombocythemia (ET) in 1990. Initially, she presented in 2014 for thrombocytosis evaluation, and a bone marrow biopsy revealed a hypercellular marrow with megakaryocytic hyperplasia, confirming ET. Treatment with hydroxyurea yielded variable responses. In 2021, abnormal WBC results prompted further investigation. Flow cytometry revealed myeloid left shift, increased myeloblasts, and teardrop-shaped red blood cells, confirming Post-ET myelofibrosis. Trisomy 8 and a 1:21 translocation were detected with FISH. Her condition was stabilized with Ruxolitinib (Jakafi) therapy. In July 2023, she presented with hepatosplenomegaly and multiple skin nodules on her upper abdomen, back, and face. A bone marrow biopsy in January 2024 revealed a hypocellular marrow, severe reticulin fibrosis (MF-3), and >20% blasts, indicating progression to acute myeloid leukemia. Flow cytometry revealed an aberrant CD34-positive blast population expressing several monocytic markers. Subsequent histopathological examination of the nodules demonstrated perivascular infiltration of immature mononuclear cells resembling blasts immunoreactive to MPO, CD117, CD163, and C68 surface markers, confirming leukemia cutis. In February 2024, the patient began treatment with venetoclax and completed one cycle of Azacitidine. Due to an inadequate response, Azacitidine was switched to Decitabine combined with Cedazuridine (Inqovi) and low-dose Jakafi. Despite undergoing six cycles of Inqovi, the patient remains unresponsive to treatment.

Conclusion: This case report illustrates the rare clinical progression of essential thrombocythemia (ET) into acute myeloid leukemia (AML), manifesting as leukemia cutis. The transition from ET to AML, while uncommon, is frequently associated with unfavorable cytogenetics and a poor prognosis. This emphasizes that close monitoring, early identification, and appropriate intervention are critical in the management of leukemic transformation.