**Core Concept**
Postoperative radiotherapy is a treatment approach used to eliminate any remaining cancer cells after surgery for endometrial cancer. It involves the use of radiation therapy to target specific areas of the body where cancer cells may be present. The indications for postoperative radiotherapy in endometrial cancer are based on various factors, including the stage and grade of the cancer, the presence of high-risk features, and the patient's overall health.

**Why the Correct Answer is Right**
Postoperative radiotherapy is often recommended for patients with high-risk features, such as high-grade tumors, deep myometrial invasion, or lymphovascular space invasion. It is also indicated for patients with stage III or IV endometrial cancer, as it can help reduce the risk of recurrence and improve survival rates. The use of radiotherapy in this setting is based on the principle of eliminating microscopic disease that may not be apparent on imaging studies or physical examination.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not provided, but in general, postoperative radiotherapy is not typically indicated for low-risk endometrial cancer, such as stage I tumors with low-grade histology.
* **Option B:** This option is not provided, but postoperative radiotherapy may be considered for patients with cervical stromal invasion, a high-risk feature that may necessitate adjuvant radiation therapy.
* **Option C:** This option is not provided, but postoperative radiotherapy may be indicated for patients with positive lymph nodes, as it can help reduce the risk of lymph node recurrence.

**Clinical Pearl / High-Yield Fact**
A key consideration in the decision to use postoperative radiotherapy for endometrial cancer is the presence of high-risk features, such as high-grade tumors or deep myometrial invasion. Patients with these features are at increased risk of recurrence and may benefit from adjuvant radiation therapy.

**Correct Answer:** A.