**Core Concept**
Radiotherapy is a treatment modality that utilizes ionizing radiation to induce cell death in cancer cells. It is particularly effective in treating tumors that are sensitive to radiation and have a high growth fraction.

**Why the Correct Answer is Right**
The correct answer is Carcinoma of the Cervix. Radiotherapy is the treatment of choice for cervical cancer, especially in the early stages. This is because radiation can target the tumor cells within the cervix and surrounding tissues, reducing the risk of surgical complications and preserving fertility. External beam radiation therapy (EBRT) is commonly used, often in combination with brachytherapy, which involves the placement of radioactive material directly into the tumor.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while radiotherapy can be used in conjunction with surgery for other types of cancer, it is not typically the primary treatment modality.

**Option B:** This option is incorrect because while some types of leukemia may be treated with radiotherapy, it is not the primary treatment modality for this condition.

**Option C:** This option is incorrect because while radiotherapy can be used to treat some types of brain tumors, it is not the primary treatment modality for this condition.

**Clinical Pearl / High-Yield Fact**
It is essential to note that radiotherapy can have late effects, including secondary malignancies and fibrosis, so careful patient selection and dosing are crucial.

**Correct Answer:** D.