**Core Concept**
Malignant transformation in leiomyosarcoma is primarily assessed through histopathological examination. The number of mitoses, or cell divisions, is a critical factor in determining the tumor's aggressiveness and potential for metastasis.

**Why the Correct Answer is Right**
The correct answer is based on the principle that the higher the number of mitoses per high power field (HPF), the greater the tumor's malignant potential. This is because leiomyosarcoma cells exhibit uncontrolled cell division, leading to an increased risk of metastasis and poorer patient outcomes. The mitotic count is a reliable indicator of the tumor's aggressiveness, with higher counts correlating with more severe disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood vessel penetration by tumor cells is a characteristic of invasive tumors, but it does not specifically address the malignant behavior of leiomyosarcoma.
**Option B:** Tumor cells in lymphatic channels indicate lymphatic spread, but this is not the primary criterion for assessing malignant behavior in leiomyosarcoma.
**Option C:** Lymphocyte infiltration is a feature of host immune response and does not directly relate to the tumor's malignant potential.

**Clinical Pearl / High-Yield Fact**
The number of mitoses per HPF is a critical prognostic factor in leiomyosarcoma, with a threshold of 10 mitoses/10 HPF often used to differentiate between benign and malignant tumors.