123I-Sodium Iodide

Description

¹²³I-Sodium Iodide (¹²³I-NaI) is a well-established radioactive salt utilized primarily for imaging the thyroid gland, thyroid cancer cells, and metastases. This generic tracer has been a cornerstone in nuclear medicine, specifically in thyroid diagnostics. Developed decades ago, it is a key tool for evaluating thyroid function and identifying thyroid-related malignancies. Unlike its therapeutic counterpart ¹³¹I-Sodium Iodide, ¹²³I-NaI is used exclusively as a diagnostic imaging agent, specifically in Single Photon Emission Computed Tomography (SPECT).

Clinical Applications

¹²³I-Sodium Iodide is considered one of the best tracers for morphological and functional studies of the thyroid gland. Its applications include:

1. Evaluation of Thyroid Function:
   • Diagnosing and managing hyperthyroidism and hypothyroidism.
   • Assessing the morphology and activity of thyroid tissues.
2. Thyroid Cancer Diagnostics:
   • Detection of iodine-avid thyroid cancer cells and metastatic lesions.

Key features of ¹²³I-Sodium Iodide include:

• Superior photon flux: ¹²³I produces approximately 20 times more photons than ¹³¹I for the same administered dose, enhancing imaging quality.
• Lower radiation burden: The radiation dose to the thyroid is only about 1% of that from ¹³¹I, making ¹²³I a safer alternative for patients whenever available.

Dosing: The typical diagnostic dose ranges from 1 to 4 mCi, administered in the form of capsules or solutions.

Availability

¹²³I-Sodium Iodide is widely distributed by major radiopharmaceutical manufacturers, although regional variations in availability exist. Providers include:

• Cardinal Health (capsules, except the 400 µCi variant discontinued in 2013).
• FUJIFILM (RadioCap).
• GE Healthcare (approved in 1976 but discontinued in 2011).
• IBA Molecular (Curium) (I-123-S-2).
• International Isotopes Inc. (since 2020).
• Mallinckrodt (Curium) (approved in the US in 1989).
• Nihon Medi System (Iodine Capsules).

Pricing:

• In Europe, the cost is typically €200-350 per mCi.
• In the USA (2020 data), a diagnostic dose costs approximately $200.

Competition

Iodine isotopes dominate thyroid imaging due to their specificity for iodine-avid cells. However, several alternatives are available, each with distinct features and limitations:

1. ¹³¹I-Sodium Iodide:
   • Primarily used for therapeutic purposes but can be employed for imaging at low doses.
   • Emits both beta and gamma radiation, making it less optimal for pure imaging but significantly more cost-effective.
2. ¹²⁴I-Sodium Iodide:
   • A PET imaging agent offering enhanced resolution compared to SPECT tracers.
   • Limited by its long half-life and higher radiation dose to patients.

While these products provide alternatives, ¹²³I-Sodium Iodide remains the preferred tracer for pure diagnostic purposes due to its superior imaging capabilities and lower radiation exposure.

Comments and Future Considerations

Thyroid and iodine-avid cancer cells naturally concentrate iodine, enabling precise imaging through the administration of radioactive iodine. However, this process presents unique challenges:

1. Thyroid Blockade:
   • Non-thyroid-targeting radioiodinated molecules can release free iodine during metabolism, potentially being trapped by the thyroid. To prevent unintended uptake, a blockade using “cold” iodine is required prior to the administration of tracers or drugs.
2. Behavior of Related Halogens:
   • While iodine is the primary target, other halogens such as bromine (to a lesser extent) and astatine can also be trapped by the thyroid. Future developments in astatine-labeled molecules must account for this behavior, as astatine salts could play a role in the treatment of thyroid cancers.

Outlook

The high specificity and safety profile of ¹²³I-Sodium Iodide ensure its continued use in thyroid imaging, though its high production cost and limited sources remain challenges. Research into alternative imaging agents, such as ¹²⁴I for PET or emerging therapies involving astatine, may expand the diagnostic and therapeutic options available for thyroid-related conditions. Nonetheless, ¹²³I-Sodium Iodide remains an invaluable tool in both clinical and research settings.