<prompt> With regard to the malignant behaviour of leiomyosarcoma, the most important criterion is -Blood vessel penetration by tumor cellsTumor cells in lymphatic channelsLymphocyte infiltrationThe number of mitoses per high power field  <response>: The distinction of Leiomyosarcoma from Leiomyomas:It is based on the combination of: The degree of nuclear atypia Mitotic index Zonal NecrosisThe most important single criteria are the number of nuclear mitosis presentWith or without Nuclear atypia Independent of nuclear atypia The presence of ten or more mitoses per ten high power x400 fields indicates malignancy with or without cellular atypism.In presence of nuclear atypia or large cells If the tumor contains nuclear atypia or large epithelioid cells, five mitoses per ten high power fields are sufficient to justify a diagnosis of malignancy. 