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What is With regard to the malignant behaviour of leiomyosarcoma, the most important criterion is - and explain in detail?
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.