Radioactive iodine therapy for thyroid cancer treatment is indicated in selected patients with differentiated thyroid cancers, including papillary and follicular thyroid cancers. However, some patients will require a lower activity of I-131 therapy while others may require a higher activity. Understanding why different I-131 doses are used for thyroid cancer treatment can help patients gain confidence in their treatment plan and decisions. Dr. Nikunj Jain, Co-Founder and Head of Nuclear Medicine at Molecular Diagnostics and Therapy, discusses factors that influence an individual’s thyroid cancer treatment with I-131. What Is I-131 Therapy for Thyroid Cancer? I-131, or radioactive iodine, is a form of iodine that can kill cancer cells that take up iodine. Normal thyroid cells take up iodine to produce thyroid hormones. Some differentiated thyroid cancer cells also retain the ability to take up iodine, allowing radioactive iodine to target iodine-avid tissue. I-131 therapy may be used in selected patients with thyroid cancer who have undergone thyroid surgery. Therapeutic use of I-131 may be indicated to destroy remaining normal thyroid tissue, cancer cells that may be present in small amounts after thyroid surgery, or to destroy cancer cells in patients with more widespread cancer spread. Radioactive iodine is not indicated in all patients with thyroid cancer, as it will only work in cancers that can take up iodine. It may be based on diagnosis, possibility of recurrence, and findings from surgical and other investigations. Why Are Different I-131 Doses Used? The main reason for different I-131 doses in thyroid cancer treatment is that each patient has different treatment goals. In some cases, a low I-131 activity may be used to remove the remaining thyroid tissue. In other instances, a higher activity may be needed to reduce the risk of cancer recurrence after surgery. Or, a larger dose might be needed if cancer has spread beyond the thyroid gland. The activity choice will also depend on the anticipated benefit for the patient in comparison with the risks of I-131 treatment. I-131 Dose for Different Treatment Goals 1. Removing Remaining Thyroid Tissue
After thyroid surgery, there may be some normal thyroid tissue left in the neck. In selected low-risk patients without evidence of cancer, a low I-131 activity may be sufficient to ablate the remaining thyroid tissue if this is indicated. This allows easier future monitoring of any cancer recurrence. In some low-risk patients, it may not even be necessary to have radioactive iodine treatment.
2. Reducing the Risk of Cancer Recurrence
There are some patients who have features of higher likelihood of cancer recurrence after thyroid surgery. These can include tumour differentiation, lymph node spread, and other findings in the thyroid and pathology reports. In this instance, the doctor may use a higher I-131 activity than that indicated for simple remnant ablation.
3. Treating Cancer That Has Spread
In patients with spread (metastases) of thyroid cancer to lymph nodes or other organs, a higher I-131 activity may be required if the cancer is still taking up iodine. The aim in such patients is to destroy any cancer cells that are found to have spread. In some patients with extensive cancer spread, additional tests such as dosimetry may be performed to help determine the most appropriate I-131 dose. The choice of I-131 activity will depend on cancer extent, likelihood of iodine uptake, prior treatment, and patient health status. What Factors Determine the I-131 Dose?
There are several factors that can influence the choice of an appropriate I-131 activity. • Cancer type and risk
The cancer type (differentiated or medullary) and pathology report, along with tumour size, risk of recurrence, and other findings, will play a part in determining if and what treatment is needed. • Amount of disease present
A patient with small amounts of thyroid tissue in the neck after surgery will have different treatment needs than someone with lymph node or distant cancer spread.
• Iodine Uptake
For I-131 to be effective, there has to be a target for the radioactive iodine to bind to. Iodine uptake is assessed with scans or other imaging, and the doctor may use these results to help determine if treatment is suitable.
• Age and general health
A patient’s age and general health, along with any other relevant medical conditions, may impact treatment options and the risks and benefits of I-131 treatment. For example, reduced kidney function could affect how radioactive iodine is dealt with in the body. • Prior treatment
Treatment received previously, such as I-131 or other cancer therapies, and the response to these treatments can also play a role in treatment selection.
• Safety considerations
In some patients, doctors may use dosimetry or other tests to help estimate the radiation dose that a patient will receive. This is especially the case in patients where a higher I-131 activity may be considered. What Do I-131 Dose Measurements Mean?
Activity of I-131 is measured in millicuries (mCi) or gigabecquerels (GBq). The number reflects the amount of radioactivity in the dose, not the amount of radiation that will be delivered to a particular tumour. For example, clinical practice guidelines indicate ranges for the activity of I-131 for remnant ablation, adjuvant treatment, and known metastases. These are general indications, and the selection will always be based on the individual patient and the recommendation of the doctor.
It will be the responsibility of the nuclear medicine specialist and treating team to recommend the most appropriate activity for the individual patient. Note that a diagnostic scan is not the same as treatment with radioactive iodine. A diagnostic scan will involve a much lower activity of I-131 and has a different intention than destroying cancer cells with higher I-131 activity. Conclusion
There are many considerations in determining the most appropriate I-131 activity for a patient with thyroid cancer. It is not as simple as selecting a low dose or high dose of I-131. The type and spread of cancer, treatment goals, previous investigations, patient health, and patient preferences have to be considered. The doctor will weigh up the risks and benefits of I-131 treatment for the individual patient. At Molecular Diagnostics and Therapy, Dr. Nikunj Jain, Co-Founder and Head of Nuclear Medicine, offers expert insight and treatment for patients who require appropriate assessment for differentiated thyroid cancer. If a patient is considering radioactive iodine therapy or has been prescribed I-131 treatment, they should ask the doctor for the reason for the selected activity, anticipated benefits and side effects, and what to do next. It is important not to compare one’s own treatment with that of another patient and to seek medical advice if there are concerns about the activity prescribed.