**Question:** For radiotherapy, an isotope is paced in or around the cancer site. It is called as:

A. Radioactive Iodine
B. Cobalt-60
C. Xenon-137
D. Iodine-131

**Correct Answer:**

**Core Concept:** Radiotherapy is a cancer treatment method that uses ionizing radiation to damage the DNA in rapidly dividing cancer cells, causing cell death and reducing tumor growth. In this context, an isotope is a radioactive substance with the same chemical properties as the naturally occurring form of the element.

**Why the Correct Answer is Right:**

* **B. Cobalt-60 (Co-60):** Cobalt-60 is a radioactive isotope of cobalt with a half-life of 5.27 years. It emits gamma rays, which are suitable for both internal and external radiotherapy applications. Placing Cobalt-60 in or around the cancer site allows targeted radiation exposure to kill cancer cells while minimizing damage to surrounding healthy tissue.

* **D. Iodine-131 (I-131):** Iodine-131 is an iodine isotope with a half-life of 8 days. It is mainly used in thyroid cancer treatment due to its affinity for the thyroid gland. While I-131 is not suitable for radiotherapy, it is used in radioiodine therapy for hyperthyroidism and thyroid cancer treatment.

**Why Each Wrong Option is Incorrect:**

* **A. Radioactive Iodine (I-131):** As explained earlier, I-131 is mainly used for thyroid-specific treatments like hyperthyroidism and thyroid cancer. It is not suitable for radiotherapy.
* **C. Xenon-133 (Xe-133):** Xenon-133 is a noble gas isotope with a half-life of 5.25 days. It emits gamma rays, making it unsuitable for radiotherapy due to its non-targeted radiation emission.

**Core Concept:** In radiotherapy, the choice of an isotope depends on its half-life, radiation type, and target tissue affinity. Gamma rays are suitable for internal radiotherapy, like I-131, while beta-emitting isotopes are suitable for external beam radiotherapy.

**Why Each Wrong Option is Incorrect:**

* **C. Xenon-133 (Xe-133):** Xenon-133 is a noble gas isotope with a half-life of 5.25 days. Its gamma rays make it unsuitable for radiotherapy due to its non-targeted radiation emission.
* **A. Radioactive Iodine (I-131):** As mentioned earlier, I-131 is used for thyroid-specific treatments like hyperthyroidism and thyroid cancer. It is not suitable for radiotherapy due to its targeted gamma rays emission.

**Clinical Pearls:**

* **Gamma rays are suitable for internal