**Core Concept**
Radiotherapy uses radioactive emissions to kill cancer cells by damaging their DNA. The type of radiation used in radiotherapy depends on the depth and location of the tumor. Alpha particles are high-energy, charged particles that can travel short distances but have a high ionizing effect.

**Why the Correct Answer is Right**
Alpha particles are used in radiotherapy for certain types of cancer, particularly those that are superficial or located in the brain. This is because alpha particles can be delivered directly to the tumor site using brachytherapy, where a radioactive source is placed near the tumor. Alpha particles have a high linear energy transfer (LET), which means they can cause significant damage to the DNA of cancer cells. The high LET of alpha particles makes them effective against cancer cells, but they are not suitable for treating deeper tumors due to their limited range.

**Why Each Wrong Option is Incorrect**
**Option A:** Gamma rays are high-energy electromagnetic radiation that can penetrate deeper into tissues than alpha particles, but they are not typically used in radiotherapy due to their low LET and high energy, which can cause damage to surrounding healthy tissues.

**Option C:** Beta particles are also charged particles, but they have a lower energy and shorter range than alpha particles. They are not typically used in radiotherapy due to their lower ionizing effect.

**Option D:** X-rays are a type of electromagnetic radiation, but they are not typically used in radiotherapy due to their low LET and inability to penetrate deep tissues.

**Clinical Pearl / High-Yield Fact**
In radiotherapy, the type of radiation used depends on the tumor's location and depth. Alpha particles are effective for superficial tumors, while gamma rays and X-rays are better suited for deeper tumors.

**✓ Correct Answer: B. Alpha particles**