**Core Concept**
Radiotherapy is a treatment modality that utilizes ionizing radiation to kill or control cancer cells. It is often used in conjunction with surgery and chemotherapy to treat various types of cancers.

**Why the Correct Answer is Right**
The correct answer is **A. Chordoma**. Chordomas are rare malignant tumors that arise from the notochord, a remnant of the embryonic structure that forms the spine and spinal cord. They are typically located in the sacrum, clivus, or cervical spine, and are known for their slow growth but high recurrence rate after surgery. Radiotherapy is often used as an adjuvant treatment to control local recurrence and improve survival rates in patients with chordomas.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because while radiotherapy is used in the treatment of osteosarcoma, it is not the primary or preferred treatment modality. Chemotherapy and surgery are typically the mainstay of treatment for osteosarcoma.

**Option C:** This option is incorrect because radiotherapy is not the treatment of choice for glioblastoma multiforme, a type of primary brain tumor. Chemotherapy and surgery are the primary treatment modalities, with radiotherapy often used in combination to improve outcomes.

**Clinical Pearl / High-Yield Fact**
Chordomas are often referred to as "horseshoe-shaped" tumors due to their characteristic radiographic appearance on imaging studies. This unique shape is a result of the tumor's origin from the notochord, which gives it a distinct appearance on plain radiographs and CT scans.

**Correct Answer:** A. Chordoma