**Question:** Tumour most sensitive to radiotherapy is

A. Leukemia
B. Brain tumor
C. Lymphoma
D. Thyroid cancer

**Core Concept:** Radiotherapy involves the use of ionizing radiation to damage the DNA of cancerous cells, ultimately leading to cell death and tumor regression. Radiotherapy is most effective against rapidly dividing cells, as the higher the cell division rate, the more likely the tumor is to be sensitive to radiation-induced DNA damage and apoptosis (cell death).

**Why the Correct Answer is Right:** Leukemia, a group of cancers involving blood-forming cells in the bone marrow, is characterized by rapid cell division. Due to this high rate of cell division, leukemia cells are particularly sensitive to radiation-induced DNA damage and cell death.

**Why Each Wrong Option is Incorrect:**

B. Brain tumor (e.g., glioblastoma) is sensitive to radiotherapy, but the correct answer is not limited to brain tumors. The focus here is on the sensitivity to radiation, not the tumor type.

C. Lymphoma is another type of blood cancer, but the correct answer is not limited to lymphomas. It is the rapid cell division rate that makes a tumor sensitive to radiotherapy, not the specific type of cancer.

D. Thyroid cancer, although sensitive to radiotherapy in some cases, is less sensitive compared to leukemia due to its slower rate of cell division.

**Clinical Pearl:** Radiotherapy is often used in conjunction with chemotherapy for leukemia treatment, as both modalities can target rapidly dividing cells and improve the overall response rate and survival.

**Correct Answer:** Leukemia (A)

**Why Leukemia is the Correct Answer:** Leukemia exhibits a high rate of cell division, making its cells particularly susceptible to radiation-induced DNA damage and cell death. This high sensitivity to radiation is crucial for the successful treatment of leukemia with radiotherapy, often in combination with chemotherapy.