How Clinical Hypothyroidism Was Defined in a Proton vs Photon Breast RT Study

At the 2026 American Society for Radiation Oncology (ASTRO) Annual Meeting, Nikolai Strusberg, MD, a radiation oncology research fellow in the Department of Radiation Oncology at Herbert Wertheim Cancer Institute, Baptist Health South Florida, spoke with CancerNetwork® about the design of a retrospective study evaluating clinical hypothyroidism after supraclavicular irradiation for breast cancer with intensity-modulated proton therapy (IMPT) vs photon-based intensity-modulated radiation therapy (IMRT).

Strusberg explained that rather than relying on laboratory values, investigators predefined a clinical end point of recommendation or initiation of thyroid hormone replacement therapy, since hypothyroidism can develop years after treatment. The median follow-up was 41.4 months. He also described how the findings should inform modality selection, noting that the decision is made on an individual basis, such as for a patient with left-sided disease who may benefit from reduced dose to the heart, lungs, thyroid, and skin, and that the overarching goal is to keep thyroid dose below thresholds that minimize the risk of clinical hypothyroidism.

Transcript:

CancerNetwork: How did you define and track clinical hypothyroidism in your study?

Strusberg: This was a retrospective study where our clinical end point was defined as recommendation or initiation of thyroid hormone replacement therapy. To our knowledge, this is the largest cohort and the longest follow-up of any series, with a median follow-up of 41.4 months. Instead of looking at labs during follow-up, we predefined a clinical end point, because these patients could develop this toxicity down the road with longer follow-up, which is something we noticed. If patients came back for a consultation in later years with newly diagnosed hypothyroidism, that was our clinical end point.

CancerNetwork: How should these findings factor into the choice between proton and photon therapy?

Strusberg: This is an individual, case-by-case analysis. Let me give an example. If you have a patient with left-sided breast cancer, you have proton availability, and that patient is going to benefit from an overall reduction in toxicities to the heart, the lungs, the thyroid, and the skin, then we could talk about it. The main goal should not be one modality or the other; it should be achieving an adequate threshold dose to the thyroid to minimize the risk of clinical hypothyroidism down the road.

Reference

Strusberg N, Herrera R, Wang D, et al. Does intensity modulated proton therapy reduce clinical hypothyroidism after supraclavicular irradiation for breast cancer? Presented at: 2026 ASTRO Annual Meeting; September 26–30, 2026; Boston, MA. Poster 2831.