**Core Concept**
Iridium-192 is a radioactive isotope used in brachytherapy for treating various types of cancer. It emits gamma radiation to destroy cancer cells. Teletherapy, on the other hand, involves external beam radiation therapy, where a radiation source is placed outside the body to target cancer cells.

**Why the Correct Answer is Right**
Iridium-192 is a suitable choice for both brachytherapy and teletherapy due to its high gamma radiation output and short half-life, which allows for effective cancer treatment with minimal damage to surrounding healthy tissues. In brachytherapy, Iridium-192 is used in seeds or needles to deliver radiation directly to the tumor site. In teletherapy, it can be used in linear accelerators to generate high-energy beams for external radiation therapy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Caesium-137 is another radioactive isotope used in brachytherapy but is not commonly used in teletherapy due to its lower gamma radiation output and longer half-life.
* **Option B:** Radium-226 is primarily used in historical brachytherapy applications and has largely been replaced by Iridium-192 due to its longer half-life and lower gamma radiation output.
* **Option C:** Cobalt-60 is a radioactive isotope used in teletherapy but is not commonly used in brachytherapy due to its lower gamma radiation output and longer half-life.

**Clinical Pearl / High-Yield Fact**
Iridium-192 is a key component in many radiation oncology applications, and its precise handling and storage are crucial to prevent radiation exposure and ensure effective cancer treatment.

**Correct Answer: C. Cobalt-60 is a radioactive isotope used in teletherapy but is not commonly used in brachytherapy due to its lower gamma radiation output and longer half-life.