## **Core Concept**
The treatment of thyroid cancer using radioactive iodine (I-131) exploits the ability of thyroid cells, including thyroid cancer cells, to uptake iodine. This method is based on the principle that thyroid cells have a specific mechanism for iodine uptake, which can be utilized to deliver a lethal dose of radiation directly to the thyroid cells.

## **Why the Correct Answer is Right**
The correct answer, **β-particles**, is right because I-131 emits both beta (β) particles and gamma (γ) rays. β-particles are high-energy electrons that travel a short distance and are primarily responsible for the therapeutic effect of I-131 in killing thyroid cancer cells. They cause damage to the DNA of the cells, leading to cell death. The short range of β-particles helps to minimize damage to non-target tissues.

## **Why Each Wrong Option is Incorrect**
- **Option A:** α-particles are not emitted by I-131. α-particles are large, doubly charged particles that have a very short range in tissues and are not typically used for therapeutic purposes in this context.
- **Option B:** γ-rays (or gamma radiation) are emitted by I-131 and are useful for imaging purposes (e.g., whole-body scans to detect iodine-avid metastases), but they are not the primary mechanism by which thyroid cancer cells are killed. γ-rays have a longer range and can exit the body, making them useful for diagnostic imaging but less effective for localized cell killing.
- **Option D:** Auger electrons are also emitted by some radioactive isotopes and can play a role in cell killing, especially for cells that take up the isotope directly into their nuclei. However, for I-131 therapy, the primary mechanism of cell killing is through β-particles.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the effectiveness of I-131 therapy depends on the ability of thyroid cancer cells to uptake iodine, which is often enhanced by prior thyroid hormone withdrawal or the use of recombinant human TSH (rhTSH) to stimulate iodine uptake. This therapy is a targeted approach that aims to minimize damage to non-thyroid tissues.

## **Correct Answer:** B. β-particles.