Radiation Expert Highlights Role of Radioactive Iodine in Differentiated Thyroid Cancer

Jimmy J. Caudell, MD, PhD

Jimmy J. Caudell, MD, PhD

For patients diagnosed with thyroid cancer, the current standard treatment involves radioactive iodine (I-131) therapy. In particular, this therapy has played a major role in the treatment of patients with differentiated thyroid cancer (DTC), with ongoing clinical trials aiming to further define the optimal use of this treatment for select groups of patients.

These trials are looking at lower-risk patient populations with DTC, particularly those with smaller tumors or limited high-risk features such as lymphovascular invasion or presence of lymph nodes. These studies aim to answer 2 major questions in regard to radioactive iodine therapy: whether or not this therapy should be given to these patients and what the dose should be.

Ongoing clinical trials are also investigating different options for patients who become refractory to radioactive iodine, including the potential use of tyrosine kinase inhibitors (TKIs).

In an interview withTargeted Oncology,Jimmy J. Caudell, MD, PhD, a radiation oncologist at Moffitt Cancer Center, discussed the role of radioactive iodine therapy in patients with DTC. He also highlighted areas of research that may help determine the best treatment regimens and doses for select patients.

TARGETED ONCOLOGY:What is the current role of radioactive iodine therapy in thyroid cancer?

Caudell: Radioactive iodine continues to play a major role in advanced DTC. Low risk thyroid cancer does not require I-31 as there is lack of evidence for overall survival or progression free survival benefit. Observational data, on the other hand, suggests benefit to patients with gross extrathyroidal extension and those with distant metastases. The problem begins when high risk patients lose the ability to concentrate radioiodine within their tumors.

TARGETED ONCOLOGY:How do you see the role of radioactive iodine therapy evolving as we move forward?

Caudell: The problem in advanced thyroid cancer is that it often has lost the ability to concentrate radioiodine, or becomes de-differentiated. In the future, there may be more successful strategies on re-differentiation therapy prior to treated with radioiodine. Thus far, clinical trials to attempt re-differentiation therapy for thyroid cancer have shown mixed results.  As we move forward molecular characterization may better identify those patients who will or will not benefit from the radioiodine earlier or later in their disease course.

TARGETED ONCOLOGY:What are the main challenges associated with radioactive iodine therapy?

Caudell: Number one, there is no safe level of radiation. We deliver radiation all the time, but if the cost and logistical challenges can be avoided for the patient that may not need it, then of course we would like to do that.

The second [challenge] is for those that could potentially benefit, we want to provide them an effective therapy at the lowest dose possible.

Since this radiation therapy is taken into the body, patients have to follow precautions that other types of radiation do not require.

TARGETED ONCOLOGY:What are the options for patients who are iodine-refractory?

Caudell: Two TKIs, sorafenib and lenvatinib, are currently approved for radioiodine-refractory thyroid cancer.   Progression-free survival benefits have been shown but not yet an overall survival benefit except in a post-hoc analysis of the SELECT trial. That suggested survival benefits in patients over the age of 65.. It’s difficult to show that in DTC.

TARGETED ONCOLOGY:What is important to know about external beam radiation in patients with thyroid cancer?

Caudell: I think that there is a myth out there that external beam radiation for thyroid cancer isn’t very effective, and I think there is a reasonable base of retrospective evidence now that it can be very helpful in certain indications, such as invasion into the esophagus, invasion into the trachea, invasion into the larynx, gross residual disease, where further surgery is not possible without significant functional consequences. With improvements in technology and radiation, the side effect profile of external beam radiation is significantly less, so it’s an option that we end up using quite a bit at our center.

TARGETED ONCOLOGY:Have we seen any data to support this?

Caudell: There is primarily for those indications I mentioned, a retrospective study at MD Anderson that’s suggesting improvements in the disease control rate with external beam radiation therapy in that situation. There are datasets from Canada as well as Memorial Sloan Kettering Cancer Center that demonstrate that as well.