Essential Thrombocythemia is one of a related group of blood cancers known as “myeloproliferative neoplasms” (MPNs) in which cells in the bone marrow that produce the blood cells develop and function. I’ve posted some links to credible medically sourced websites for you which explains MPNs and ET.

From Cleveland Clinic https://my.clevelandclinic.org/health/diseases/24031-essential-thrombocythemia
~Is essential thrombocythemia a form of cancer?
“Essential thrombocythemia is an example of a myeloproliferative neoplasm. These are blood cancers that happen when your body produces too many blood cells.”

~~
BloodCancer.com https://blood-What is Essential Thrombocythemia?
cancer.com/clinical/essential-thrombocythemia
~~~
Leukemia & Lymphoma Society
https://www.lls.org/myeloproliferative-neoplasms/essential-thrombocythemia

I know the word cancer sparks a unique level of fear. The MPNs are often very slow to develop and aren’t aggressive so sometimes it feels that the term cancer is over stated. However anytime we have issues with our bone marrow not producing healthy blood cells or producing them out of control, that is a blood cancer. Blood cancer affects how your body produces blood cells and how well those cells work.
So to answer your question from credible sources, yes, ET is a blood cancer. Does this help? ☺️

According to my Heme doc, not all ETs are cancer. I have Jak2, which is not cancer and unlikely to become so. Best to ask your doc for more insight and perhaps more testing.