With regard to the malignant behaviour of leiomyosarcoma, the most important criterion is -: (A) Blood vessel penetration by tumor cells, (B) Tumor cells in lymphatic channels, (C) Lymphocyte infiltration, (D) The number of mitoses per high power field
Answer is D. 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.