In treatment of papillary carcinoma thyroid, radioiodine destroys the neoplastic cells predominantly by
**Core Concept**
Radioiodine therapy is a common treatment modality for papillary carcinoma of the thyroid, particularly after surgical resection. The mechanism of action of radioiodine involves the uptake of radioactive iodine by thyroid cells, which are then selectively destroyed by the emitted radiation.
**Why the Correct Answer is Right**
Radioiodine destroys neoplastic cells in papillary carcinoma of the thyroid primarily through the induction of DNA double-strand breaks. Thyroid cells take up iodine via the sodium-iodide symporter (NIS), a transmembrane protein that is highly expressed in thyroid tissue. Radioactive iodine (I-131) is then concentrated in the thyroid cells, where it emits beta particles that cause damage to the DNA, ultimately leading to cell death.
**Why Each Wrong Option is Incorrect**
* **Option A:** Radioiodine does not primarily destroy neoplastic cells through the induction of apoptosis. While apoptosis may occur as a result of DNA damage, it is not the primary mechanism of action of radioiodine.
* **Option B:** Radioiodine does not destroy neoplastic cells through the inhibition of the sodium-iodide symporter (NIS). In fact, NIS is responsible for the uptake of radioactive iodine by thyroid cells, allowing radioiodine to concentrate in the thyroid tissue.
* **Option D:** Radioiodine does not destroy neoplastic cells through the activation of thyroid-stimulating hormone (TSH) receptors. TSH receptors are involved in the regulation of thyroid cell growth and function, but they are not directly involved in the mechanism of action of radioiodine.
**Clinical Pearl / High-Yield Fact**
Radioiodine therapy is most effective in papillary carcinoma of the thyroid when the tumor has taken up radioactive iodine, as indicated by a positive iodine scan. This is because radioiodine is selectively concentrated in thyroid cells that express the NIS, allowing for targeted destruction of neoplastic cells.
**Correct Answer: C. Induction of DNA double-strand breaks.**