Essential thrombocythaemia is a myeloproliferative disorder, as you can probably read in your leaflets; you are producing too many platelets.
Any such overproduction of cells can loosely be described as a type of cancer, although that is not a particularly helpful label in this condition which is only slowly progressive and relatively benign.
The hydroxycarbamide will keep everything in control, and you very likely will be attending the Haematology Department for many years/decades to have your blood tested, so I shouldn't worry too much.
You might find this link useful.
http://www.mpdvoice.org.uk/