**Core Concept**
Postoperative radiotherapy is a treatment strategy used to eliminate residual cancer cells after surgical removal of a tumor. The decision to use postoperative radiotherapy depends on various factors, including the type and stage of cancer, the surgical margin status, and the patient's overall health. In some malignancies, the role of postoperative radiotherapy is limited due to the tumor's biology and the effectiveness of other treatment modalities.

**Why the Correct Answer is Right**
The correct answer is **Medulloblastoma**. Medulloblastoma is a highly malignant brain tumor that primarily affects children. It is sensitive to radiation and chemotherapy, and surgery is often curative. Postoperative radiotherapy is not commonly used for medulloblastoma due to the risk of long-term cognitive and neurological sequelae. The primary treatment approach for medulloblastoma involves maximal surgical resection followed by chemotherapy and craniospinal irradiation in selected cases.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because postoperative radiotherapy is commonly used in the treatment of **Glioblastoma**, a more aggressive and resistant brain tumor.
* **Option B:** This option is incorrect because postoperative radiotherapy is often used in the treatment of **Rectal Cancer**, particularly for patients with high-risk features such as positive margins or lymph node involvement.
* **Option C:** This option is incorrect because postoperative radiotherapy may be used in the treatment of **Osteosarcoma**, a primary bone tumor, especially in cases with high-risk features such as lung metastases.

**Clinical Pearl / High-Yield Fact**
Medulloblastoma is the most common malignant brain tumor in children, accounting for approximately 20% of all pediatric brain tumors. It is essential to differentiate medulloblastoma from other brain tumors, such as glioblastoma, which requires a more aggressive treatment approach.

**Correct Answer:** D.