## **Core Concept**
The question tests knowledge of radiation types used in radiotherapy. Radiotherapy, or radiation therapy, is a treatment method that uses ionizing radiation to destroy or damage cancer cells. The most common types of radiation used are **ionizing radiations**, which include X-rays, gamma rays (γ rays), alpha particles (α rays), and beta particles (β rays).

## **Why the Correct Answer is Right**
X-rays and gamma rays (γ rays) are the most commonly used forms of radiation in radiotherapy due to their ability to penetrate tissues and cause damage to cancer cells. **X-rays** are produced by accelerating electrons, which then collide with a metal target, producing X-ray photons. **Gamma rays** are naturally emitted by radioactive isotopes, such as cobalt-60 or cesium-137, and have a higher energy level than X-rays, allowing for deeper penetration. Between X-rays and γ rays, X-rays are more frequently used due to their accessibility and the ease with which their energy can be adjusted for different treatment depths.

## **Why Each Wrong Option is Incorrect**
- **Option C: α rays** - Alpha particles are not commonly used for external beam radiation therapy because they have a very short range in tissue and can be stopped by a sheet of paper or the outer layers of human skin, making them unsuitable for treating tumors inside the body. However, they are used in certain types of brachytherapy.
- **Option D: β rays** - Beta particles have a slightly longer range than alpha particles but still cannot penetrate far into tissue. Like alpha particles, they have limited application in external beam radiation therapy but are used in certain brachytherapy treatments and in some cases of superficial radiotherapy.
- **Option c) γ rays** and **Option a) X rays** are both correct types of radiation used in radiotherapy; however, the question format suggests selecting the best answer which encompasses both.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the choice of radiation type depends on the tumor's location, size, and depth, as well as the patient's overall health. **X-rays** are particularly versatile and widely available, making them a common choice for many radiation therapy treatments.

## **Correct Answer:** A. a) X rays