Nuclear Medicine and the Thyroid Part 3
Part 3 of my review of nuclear medicine thyroid imaging and therapy for radiology board exams.
Show Notes/Study Guide:
What subtypes of thyroid cancer classically may not take up I131?
Medullary thyroid cancer and anaplastic thyroid cancer classically may not take up I131 and therefore may not respond as well to I131 therapy compared to other types of thyroid cancer. Remember the MEN 2A and 2B syndrome association with medullary thyroid carcinoma. Note that medullary thyroid cancer has neuroendocrine features and may have uptake on MIBG/octreotide imaging. Papillary thyroid cancer is typically radioiodine avid.
What other factors contribute to treatment resistance of thyroid cancer to I131 therapy?
Patients with prior I131 therapy are more likely to be resistant to I131 therapy in the future. So if you are re-treating with I131 you often have to significantly increase the I131 for subsequent therapies. Prior methimazole treatment is also thought to possibly contribute to resistance to I131 therapy. Poor patient preparation is also a cause of failure or poor response to I131 therapy.
True or False: For thyroid cancer therapy thyroidectomy surgery typically precedes thyroid treatment with I131?
True. Thyroidectomy is first performed and I131 is subsequently given to ablate any residual thyroid tissue. Note that it is not possible, even with the most skilled surgeons, to remove all thyroid tissue at surgery so there will always be a thyroid remnant that can be ablated. As there can be variation in how much thyroid tissue remains post-thyroidectomy, one can perform a thyroid uptake test with I123/I131 and if thyroid uptake is something like 5% or greater, those patients may be at risk for thyroid pain following I131 therapy and may be considered for steroid treatment during I131 therapy to reduce symptoms vs return to OR to remove additional tissue.
Do you want TSH to be elevated or low prior to I131 cancer ablation therapy?
TSH should be elevated prior to I131 ablation therapy in order to stimulate residual thyroid tissue to take up as much I131 as possible. Two primary options exist to raise TSH values. The first is cessation of thyroid hormone replacement to stimulate the natural TSH release as the body becomes hypothyroid. The second option is to remain on thyroid hormone and take recombinant TSH which is also known as thyrogen. Note that a minimum TSH value of 30 is often considered acceptable pre I131 ablation and a TSH value of 50 or higher may be considered ideal.
What are common I131 doses for thyroid cancer without nodal or distant metastatic disease, thyroid cancer with local nodal metastases and thyroid cancer with distal metastases?
I131 for contained thyroid cancer with no nodal or distant spread: 50 to 100 mCi
I131 for thyroid cancer and local nodal disease in neck: 150 mCi
I131 for thyroid cancer and distant disease: 200 mCi
What are general dose ranges for I131 therapy of Graves disease, autonomous hyperfunctioning thyroid nodule and toxic multinodular goiter to keep in mind for board exam purposes?
Graves disease is often treated with 5 to 15 mCi I131
Autonomous hyperfunctioning thyroid nodule is often treated with 15-25 mCi I131
Toxic multinodular goiter is often treated with 25-35 mCi I131
Note that calculations do exist whereby doses may be calculated based on thyroid weight and radioiodine uptake values. However, these general ranges are something that may be beneficial to know for board exam purposes.
What are common precautions that individuals must take prior to I131 therapy?
Following I131 therapy a patient must isolate for 3 days including bathroom hygiene, sleeping alone, no exposure of others to bodily fluids including saliva and urine. Additionally, patients should stay well hydrated.
What is an estimated risk of malignancy from a single cold nodule on a radioactive iodine scan?
A single cold nodule has an approximate 15 to 20% chance of malignancy. Note that risk of malignancy increases if a cold nodule is seen in younger patients, if a nodule is hard on palpation, if there is a history of neck radiation, and with family history of thyroid cancer.
What is an estimated risk of malignancy from a hot nodule on a thyroid scan?
In general, the risk of thyroid cancer in a nodule that is hot on a radioactive iodine scan is thought to be less than 1%.
Is hypothyroidism common in patients with toxic multinodular goiter following I131 therapy?
No. In toxic multinodular goiter the radioactive iodine preferentially is taken up into the toxic nodule(s) and the remainder of the gland has suppressed uptake and is therefore not ablated. Note that many of these patients with toxic multinodular goiter will become euthyroid following I131 treatment. The same principle would also hold true for a hyperfunctioning thyroid nodule—the background thyroid is suppressed and the radioactive iodine is preferentially taken up within and preferentially ablates the hyperfunctioning nodule, sparing the normal thyroid parenchyma, and potentially ending up in a euthyroid state.
How can one differentiate between a Tc-99m pertechnetate vs radioactive iodine thyroid scan?
If you see higher salivary gland uptake than thyroid uptake and lots of background counts then the scan is more likely a Tc-99m scan. A radioactive iodine scan would show robust thyroid uptake with little background uptake.
What should you consider if thyroglobulin levels are rising in a patient who has completed thyroidectomy and I131 ablation for thyroid cancer and the thyroid I123/I131 scan is negative? What is the next best test to consider?
One must consider the possibility that the thyroid cancer is truly back, as suggested by the thyroglobulin levels, but has de-differentiated and therefore no longer takes up radioactive iodine. In this setting, one would want to perform an FDG-PET/CT study to show where the site(s) of recurrence are as de-differentiated thyroid cancer tends to be FDG avid.
What type of collimator is best for I131 imaging?
High energy collimators are used for I131 imaging given the very high 364 keV energy. If lower energy collimators are used you would expect to see septal penetration with the star-like artifact that is classic for this entity. Note that I131 is basically the highest energy radioisotope commonly used in general nuclear medicine
Besides thyroid imaging can you name another nuclear medicine scan that uses I123/I131?
MIBG uses I123/I131. Note that one needs to block the thyroid gland before MIBG to prevent thyroid uptake. Lugols Iodine solution is commonly used prior to imaging to prevent unnecessary radiation to the thyroid when using MIBG imaging. Remember to block the thyroid first prior to MIBG imaging. A dual tracer technique for parathyroid imaging can also be performed where I123 is given to show background thyroid uptake that is then subtracted from a Tc-sestamibi or Thal201 chloride scan that is taken up by the parathyroid glands and with some background thyroid uptake. If you subtract the I123 info you are left with uptake primarily from the parathyroid glands. I123 Ioflupane may also be used for dopamine transporter imaging aka DaTscan. Note that Octreoscan uses In111 NOT I123.
What is thyroid stunning?
Thyroid stunning is a reduction in I131 uptake below predicted values following a low-dose diagnostic I131 scan. The thought is that either reduced cell function and/or cellular death following a diagnostic I131 scan (such as an I131 uptake study or whole body search) can lead to reduced therapeutic I131 uptake due to “stunned” (or dead) thyroid cells secondary to the initial I131 low dose study.
Approximately how long should you wait following IV iodinated contrast administration to treat a patient with I131?
Typically about 6-8 weeks.
What are NRC guidelines for releasing a patient following I131 oral administration?
1. No individual of the public is likely to receive more than 5 mSv exposure from the patient treated with I131.
2. When a survey meter reading at 1 meter is less than 0.07 mSv/hour (7 mrem/hour)
3. When administered activity is 33 mCi or less.
Prior to release that authorized user should instruct the patient regarding best practices to minimize radiation exposure to others and written instruction describing recommended methods to limit radiation exposure to others is generally advised. Precautions include having the patient sleep alone, avoid pregnant individuals and children, avoid public transportation, follow strict bathroom hygiene, etc. Note also that I131 exposure is at high risk through bodily fluids but not through touching object. For example, a family member need not worry if they touched a door handle that was recently touched by the person treated with I131.
Want more info?
Check out this RadioGraphics article: https://pubs.rsna.org/doi/full/10.1148/rg.234025716
Good overview: https://www.ncbi.nlm.nih.gov/books/NBK559283/
SNMMI I131 therapy practice parameters: https://s3.amazonaws.com/rdcms-snmmi/files/production/public/docs/I-131_V3.0_JNM_pub_version.pdf
