With regard to the malignant behaviour of leiomyosarcoma, the most important criterion is -

The distinction of Leiomyosarcoma from Leiomyomas: It is based on the combination of:  The degree of nuclear atypia  Mitotic index  Zonal Necrosis The most important single criteria are the number of nuclear mitosis present With 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.