<prompt> With regard to the malignant behavior of leimyosarcoma, the most impoant criterion is:Blood vessel penetration by tumor cellsTumor cells in lymphatic channelsLymphocyte infiltrationThe number of mitoses per high power field  <response>: An impoant microscopic criterion for distinguishing leiomyosarcoma from leiomyoma is the number of mitoses present. Greater than 10 mitoses per10 high-power fields hpf is associated with frank malignancy leiomyosarcoma. Ref: Robbins Basics of Pathology , 7th Edition, Page I090; Oncology: An Evidence-Based Approach By Alfred E. Chang, 2006, Page 944. 