A neoplasm arising from hematopoietic cells found in the bone marrow, peripheral blood, lymph nodes and spleen (organs of the hematopoietic system). Hematopoietic cell neoplasms can also involve other anatomic sites (e.g. central nervous system, gastrointestinal tract), either by metastasis, direct tumor infiltration, or neoplastic transformation of extranodal lymphoid tissues. The commonest forms are the various types of leukemia, Hodgkin and non-Hodgkin lymphomas, myeloproliferative neoplasms, and myelodysplastic syndromes.