I-131 scan is a non-invasive radionucleotide scan used for imaging of functional thyroid tissue and thyroid cancer remnant/metastasis. Theranostics in nuclear medicine described as combining diagnostic imaging and treatment for a specific disorder by using the same molecule either radiolabeled in a different form or same radiolabeled agent in different doses. The visualization of the target allows for precision therapy. Iodine molecules can be labeled by radiotracers and used for diagnostic and treatment purposes in thyroid disorders.
The thyroid gland produces thyroid hormones, which are essential to survive and modulate energy metabolism. Iodine is required for thyroid hormone production. Ingested iodine is selectively taken up by thyroid follicular cells with the help of sodium-iodine symporters (NIS) to be processed. NIS is regulated by TSH (thyroid-stimulating hormone). A circulating form of iodine is called iodide, and once it enters follicular cells, it is oxidized by thyroid peroxidase (TPO) enzyme to form iodine. Organification is the next biochemical reaction of the integration of iodine into the thyroglobulin to generate thyroid hormones, mainly thyroxine (T4) and triiodothyronine (T3).
Thyroid stunning can occur during a radionucleotide scan. It is defined as a suppression of iodine production from thyroid, which is temporary. It occurs due to radiation given off the scan from I-131, thus trapping function of the thyrocytes and thyroid cancer cells. It is a radiological phenomenon that a clinician needs to be aware of when administering radioactive iodine. When a small dose of I-131 radioactive iodine given prior to a large dose for treatment, there could be a “stunning” of thyroid cells, which may decrease the effectiveness of treatment.
The two most common radiotracers used in thyroid scans are I-123 and I-131. The differences between these two isotopes are as follows:
- I-123 has a short half-life (approximately 13 hours), decays by gamma emission, has better imaging quality, less stunning effect, expensive, and less readily available.
- I-131 has a longer half-life (approximately 8.2 days), decays by beta emission, is cheaper, readily available, but may have a potential stunning effect due to higher energy capture, allows delayed imaging.
I-131 scan is a powerful tool to investigate several thyroid disorders but is mainly used to:
- To characterize thyroid function in the case of hyperthyroidism, subclinical hyperthyroidism, multinodular goiter, toxic nodule, thyroiditis, congenital hypothyroidism.
- To visualize ectopic thyroid tissue.
- To visualize normal or/cancerous thyroid tissue remnant, thyroid cancer recurrence or distant metastasis
- To re-stage thyroid cancer 6-12 months after thyroidectomy and radioactive iodine ablation therapy.
- To help with the decision of I-131 therapy dose (diagnostic pre-treatment whole-body scan)
- To identify the I-131 therapy dose that is safe for vital organs (in a patient who has been treated multiple times before). This procedure called dosimetry.
Normal and Critical Findings
Accurate interpretation of the I-131 scan is essential to identify the correct clinical course of action.
I-131 whole body scan counts on the fact that thyroid tissue has a high affinity to uptake and trap the Iodine, which is performed by NIS. Some extra-thyroidal tissues like stomach, salivary glands, breast (especially if lactating), and urinary tract also express NIS and show physiologic uptake. Similarly, as absorbed radioiodine is metabolized in the liver and excreted in the urinary tract, visualization of the liver, gall bladder, kidney, bladder, and ureters is common.
Ectopic thyroid tissue may be an embryological remnant or malformation during the development that can be captured with an I-131 scan. Common examples are lingual or sublingual thyroid, thyroglossal duct, or a mediastinal thyroid gland. Ectopic thyroid tissue in ovaries is called struma ovarii.
In a patient who has undergone thyroidectomy, any uptake apart from physiologic distribution on a post-therapy I-131 scan may be considered as a remnant (if localized in the thyroid bed after the first I-131 ablation/therapy) or metastasis (out of the thyroid bed). Post-therapy whole-body scan is very valuable and may change the plan of care with additional information.
False-positive Lesions: Inflammatory, cystic, non-thyroid neoplastic disorders, for example, pleural or pericardial effusions; aspergilloma, arthritis; ovarian or breast cysts. NIS expression of the tumor and increased blood flow resulting in iodine uptake of some malignancies has been reported as in breast cancer, gastric adenocarcinoma, or bronchial adenocarcinoma. External contamination by body fluids or rarely during the intake of the Iodine may imitate metastatic involvement. Usually, the pattern is recognized easily by experienced specialists, and repeat imaging after decontamination clarifies the case. They are almost always superficial, and obtaining lateral and/or oblique images helps to identify the lesion. In addition, utilizing SPECT-CT offers very valuable input about the anatomical location of the uptake for further evaluation. This is why it is important for the surgeon to remove all of the thyroid tissue at surgery, as even a little bit of thyroid tissue left will render the test and treatment inaccurate.