**Core Concept**
Emergency radiotherapy is a critical intervention in certain clinical scenarios, where immediate radiation therapy is necessary to alleviate severe symptoms or prevent life-threatening complications. This concept is rooted in the principles of radiation oncology, where precise and timely delivery of radiation is essential to achieve optimal outcomes.

**Why the Correct Answer is Right**
Emergency radiotherapy is typically given in cases of spinal cord compression due to malignancy. The mechanism involves the rapid progression of cancer, which can compress the spinal cord, leading to severe neurological deficits or even paralysis. In such situations, emergency radiotherapy is administered to rapidly shrink the tumor and alleviate pressure on the spinal cord. This is often done in conjunction with corticosteroids to reduce inflammation and edema. The goal is to prevent further neurological deterioration and preserve function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because emergency radiotherapy is not typically given for primary brain tumors, as the primary treatment approach involves surgical resection and adjuvant chemotherapy or radiotherapy, not emergency radiotherapy.
**Option B:** This option is incorrect because emergency radiotherapy is not given for metastatic bone pain, where the primary treatment involves palliative measures such as analgesics, bisphosphonates, and radiation therapy, but not emergency radiotherapy.
**Option C:** This option is incorrect because emergency radiotherapy is not given for pelvic malignancies, where the primary treatment involves surgery, chemotherapy, and radiation therapy, but not emergency radiotherapy.

**Clinical Pearl / High-Yield Fact**
In cases of spinal cord compression due to malignancy, emergency radiotherapy should be initiated promptly, ideally within 24 hours, to prevent further neurological deterioration and preserve function.

**Correct Answer:** D.