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Radioactive Iodine Therapy Dosimetry for a Case of Functioning Follicular Thyroid Carcinoma Metastases

This case report demonstrates that using Iodine-124 PET/CT dosimetry to guide the administration of Radioactive Iodine-131 therapy enables the safe delivery of a high radiation dose to functioning follicular thyroid carcinoma metastases, resulting in a significant reduction in serum thyroglobulin and effective disease management with minimal toxicity.

Original authors: Christopher James Kleimeyer, Sarah Ong, Shaun Patford, Matthew Griffiths, Lauren Burrage, Samuel Kyle, David Pattison, Tahleesa Cuda, Roslyn Francis

Published 2026-07-27

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CC BY 4.0

Original authors: Christopher James Kleimeyer, Sarah Ong, Shaun Patford, Matthew Griffiths, Lauren Burrage, Samuel Kyle, David Pattison, Tahleesa Cuda, Roslyn Francis

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the human body as a bustling city, and inside that city, there are specialized factories called glands. One of these, the thyroid, sits in the neck and acts like a thermostat, regulating how fast the city's energy is burned. Sometimes, however, a factory can go rogue. In a rare and tricky scenario, a cancer called follicular thyroid carcinoma doesn't just sit in the neck; it packs its bags and moves to other parts of the city, like the lungs or bones. Even stranger, these runaway factories keep working, pumping out hormones that make the patient's heart race and body overheat, a condition known as hyperthyroidism.

To fight this, doctors have a powerful weapon: Radioactive Iodine (RAI). Think of iodine as a specific key that only thyroid cells (both the good ones and the cancerous ones) know how to use. When you give a patient a dose of radioactive iodine, the cancer cells greedily swallow it up, thinking it's food. Once inside, the radiation acts like a tiny, targeted bomb, blowing the cancer cells from the inside out. But here's the catch: if you give too little, the cancer survives; if you give too much, you might accidentally hurt the patient's bone marrow or lungs. For a long time, doctors have played it safe, guessing a "standard" dose that works for most people. But for a patient with these super-active, roaming cancer factories, a guess might not be enough. You need a map.

This paper tells the story of a 74-year-old woman with exactly this kind of challenging case. She had follicular thyroid cancer that had spread to her lungs, bones, and even her scalp, and it was so active it was causing severe hyperthyroidism. Instead of guessing her treatment dose, the medical team at the Royal Brisbane and Women's Hospital decided to use a high-tech "scout mission" first. They gave her a tiny, harmless amount of a special type of iodine called Iodine-124 and used a PET/CT scanner to watch how it moved through her body over several days. It was like sending a spy drone to see exactly how much fuel the cancer was hoarding and how long it held onto it.

The results of this scout mission were eye-opening. Because her cancer was so active, it was holding onto the iodine for a very long time, recirculating it through her blood. The team used this data to calculate the "Maximum Tolerable Activity" (MTA)—the biggest dose of the real radioactive treatment (Iodine-131) she could safely handle without hurting her blood-making factories (bone marrow). Their calculations suggested she could safely take 3,100 MBq of the treatment, which would deliver a massive 217 Gy (a unit of radiation dose) to a specific spot in her lung. To be safe, they actually gave her 3,000 MBq.

The outcome was a resounding success. Six months after the treatment, her "tumor marker" levels (a substance in the blood that tells you how much cancer is present) dropped dramatically from a huge 21,148 µg/L down to 4,610 µg/L. The intense heat and racing heart caused by the cancer stopped, allowing doctors to switch her from heavy medication to a standard thyroid hormone replacement. She is now feeling much better, with only mild hip pain remaining, and the team plans to give her a second dose in six months.

The authors conclude that for patients with these rare, super-active cancers, using this "scout mission" (Iodine-124 PET/CT) to plan the dose is a smart move. It allows doctors to hit the cancer with the maximum possible punch while keeping the patient safe, rather than relying on a one-size-fits-all guess. It's a reminder that in medicine, sometimes the best way to treat a complex problem is to measure it first.

Technical Summary: Radioactive Iodine Therapy Dosimetry for Functioning Follicular Thyroid Carcinoma Metastases

Problem Statement
Metastatic differentiated follicular thyroid cancer (FTC) with functioning metastases presents a rare and clinically challenging scenario. These patients often suffer from persistent hyperthyroidism (thyrotoxicosis) due to the large total tumor mass and the presence of thyroid-stimulating immunoglobulins, even after total thyroidectomy. While Radioactive Iodine (RAI) therapy using Iodine-131 ( $^{131}$ I) is the standard treatment for such metastatic disease, determining the optimal administered activity is difficult. The presence of functioning metastases leads to unpredictable and prolonged whole-body retention of activity due to $^{131}$ I metabolism and recirculation. Consequently, most clinical practice relies on fixed or empiric dosing (typically 1100–3700 MBq). However, this approach risks suboptimal tumor dosing or excessive toxicity. Specifically, administering non-maximum activities may allow less differentiated cells to survive, potentially inducing dedifferentiation and reducing therapeutic efficacy over multiple courses. Conversely, without precise calculation, there is a risk of exceeding the maximum tolerable activity (MTA), leading to bone marrow toxicity, pneumonitis, or pulmonary fibrosis. Additionally, the risk of thyroid storm during treatment remains a critical safety concern.

Methodology
This retrospective case report details the management of a 74-year-old female diagnosed with functioning metastatic FTC. The patient presented with symptomatic thyrotoxicosis and widespread metastases (scalp, lungs, adrenal, rib, and sacroiliac) confirmed by FDG PET/CT and biopsy.

The core methodology involved a pre-therapeutic dosimetry protocol using Iodine-124 ( $^{124}$ I) PET/CT to guide the subsequent $^{131}$ I therapy:

Diagnostic Tracer Administration: The patient received an oral administration of 28 MBq of $^{124}$ I.
Imaging and Sampling: Multi-timepoint PET/CT imaging was performed at 6, 24, 48, 72, and 144 hours post-administration. Concurrently, blood sampling and whole-body gamma counting were conducted.
Dosimetry Calculation: The MTA was defined as the activity delivering a radiation absorbed dose of 2 Gy to the blood. Using the method described by Jentzen et al. (2015), curve fitting via piecewise trapezoidal integration was performed up to 168 hours and physically decayed to infinity to account for continuous circulation from the functional disease.
Therapeutic Administration: Based on the $^{124}$ I dosimetry, an activity of 3000 MBq of $^{131}$ I was administered (slightly below the calculated MTA of 3100 MBq) to target a specific lung metastasis index lesion with an estimated dose of 200 Gy.
Post-Therapy Monitoring: Post-treatment scintigraphy was performed at 3 and 7 days. The patient was hospitalized for three days until external dose rates permitted discharge, followed by radiation precautions for two weeks.

Key Results
The dosimetry-guided approach yielded the following outcomes:

Dose Delivery: The administered 3000 MBq of $^{131}$ I successfully delivered a high absorbed dose to the target lesion while adhering to safety limits for the blood.
Biochemical Response: Six months post-therapy, the patient's serum thyroglobulin levels decreased significantly from a pre-therapy baseline of 21,148 µg/L to 4,610 µg/L, indicating a good partial response. Thyroglobulin antibody levels remained undetectable.
Clinical Management: The patient's hyperthyroidism was managed effectively; carbimazole was discontinued one month post-treatment, and she was transitioned to thyroxine with a TSH target of <0.1 mIU/L.
Safety: The patient remained asymptomatic aside from mild left hip pain. No severe toxicity was reported, and the patient was successfully discharged with standard radiation precautions.

Significance and Claims
The paper asserts that utilizing $^{124}$ I PET/CT for dosimetry calculations is a useful approach for RAI therapy in metastatic differentiated thyroid cancer (DTC), particularly in cases involving functioning metastases. The primary significance lies in the ability to ensure that the maximum radiation absorbed dose is safely delivered to tumor lesions while minimizing off-target effects and toxicity.

The authors highlight that this method addresses the specific challenges of recirculating radioactivity in patients with functional disease, which can lead to prolonged marrow exposure. By calculating the MTA rather than relying on fixed empiric doses, clinicians can potentially avoid the gradual loss of therapeutic efficacy associated with under-dosing and the risk of tumor dedifferentiation. The case demonstrates that this dosimetric approach allows for the safe administration of high-activity therapy (3000 MBq) in a complex patient, facilitating the eventual cessation of anti-thyroid medications and the transition to TSH suppression therapy. The paper concludes that while fixed dosing remains common, dosimetry offers a tailored strategy to optimize the therapeutic window for metastatic DTC.

Technical Summary: Radioactive Iodine Therapy Dosimetry for Functioning Follicular Thyroid Carcinoma Metastases

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