Describe in your own words what internal radiotherapy and external radiotherapy are. Give an example of a commonly used radioisotope in each procedure, and explain why these radioisotopes can have quite different half-lives.

Internal radiotherapy and external radiotherapy are two types of treatments used to combat cancer.

Internal radiotherapy, also known as brachytherapy, involves placing a radioactive source inside the body, directly into or near the tumor. This allows a high dose of radiation to be delivered to the cancer cells while minimizing damage to the surrounding healthy tissues. An example of a commonly used radioisotope in this procedure is Iodine-125. It has a relatively long half-life of 59.4 days, which allows it to continuously emit radiation over an extended period, effectively killing the cancer cells.

External radiotherapy, on the other hand, involves directing a beam of radiation at the tumor from outside the body. This is typically done using a machine called a linear accelerator. The most commonly used radioisotope in this procedure is Cobalt-60. It has a shorter half-life of 5.27 years, which means it decays more rapidly and emits a higher dose of radiation in a shorter time. This is beneficial in external radiotherapy as it allows for a high dose of radiation to be delivered to the tumor in a short period, minimizing the duration of treatment.

The half-lives of these radioisotopes can vary greatly because they are chosen based on the specific needs of the treatment. A radioisotope with a longer half-life is beneficial for internal radiotherapy as it allows for a continuous emission of radiation over a longer period. Conversely, a radioisotope with a shorter half-life is beneficial for external radiotherapy as it allows for a high dose of radiation to be delivered in a shorter time.