## **Core Concept**
External beam radiotherapy uses high-energy rays to target and kill cancer cells. Radioisotopes that emit ionizing radiation, such as gamma rays or X-rays, can be used as sources for external beam radiotherapy. The choice of radioisotope depends on its physical properties, such as half-life, energy of emission, and availability.

## **Why the Correct Answer is Right**
Cobalt-60 (. ) is a synthetic radioactive isotope of cobalt with a half-life of approximately 5.3 years. It emits gamma rays with energies of 1.17 and 1.33 MeV, making it suitable for external beam radiotherapy. Cobalt-60 machines, also known as cobalt units, have been widely used for cancer treatment due to their reliability, relatively low cost, and ability to deliver a high dose of radiation to tumors.

## **Why Each Wrong Option is Incorrect**
* **Option A:** Radium-226 is a naturally occurring radioactive isotope that was historically used in brachytherapy (internal radiation therapy) for cancer treatment, not for external beam radiotherapy.
* **Option B:** Iodine-131 is primarily used for the treatment of thyroid disorders, including thyroid cancer, and is administered internally, usually orally, not as an external beam source.
* **Option D:** Cesium-137 is another radioisotope used in brachytherapy and has been used in some external beam radiation therapy units, but it is less commonly associated with external beam radiotherapy compared to Cobalt-60.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while Cobalt-60 units were once a mainstay of radiation oncology, advances in technology have led to the development of linear accelerators, which are now more commonly used for external beam radiation therapy due to their ability to produce a wider range of energies and more precise targeting of tumors.

## **Correct Answer:** .