**Core Concept**
The staging of endometrial cancer is based on the depth of invasion into the myometrium, among other factors. **Endometrial cancer** staging is crucial for determining prognosis and treatment. The **International Federation of Gynecology and Obstetrics (FIGO)** staging system is commonly used.

**Why the Correct Answer is Right**
In the FIGO staging system, the depth of myometrial invasion is a key factor. For **Stage IB**, the cancer has invaded the myometrium to a depth of less than 50% of the myometrial thickness, but specifics can vary based on the version of the staging system being used. However, a depth of invasion of 10 mm would generally be considered as not exceeding half of the typical myometrial thickness, thus fitting into an early stage.

**Why Each Wrong Option is Incorrect**
**Option A:** This is typically reserved for cancers confined to the endometrium without myometrial invasion.
**Option C:** This stage often involves deeper invasion or specific other criteria not met by 10 mm of invasion alone.
**Option D:** This would typically involve more extensive disease, including deeper invasion or spread beyond the uterus.

**Clinical Pearl / High-Yield Fact**
Remembering the key factors in the FIGO staging system, including the depth of myometrial invasion, is crucial for accurately staging endometrial cancer.

**Correct Answer:** B. IB