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Preclinical studies have shown that radiation therapy modulates antitumor immune responses. However, circulating T-cell responses after radiation therapy in patients with cancer have been poorly characterized. This study aims to explore the changes in circulating T cells after stereotactic body radiation therapy (SBRT). Peripheral blood samples of 30 patients with breast cancer who underwent SBRT for bone metastasis were analyzed using multicolor flow cytometry. Phenotypes of PD-1+ CD8+ T cells and regulatory T (T REG) cells were examined. Additionally, plasma protein levels were analyzed using a bead-based immunoassay. Circulating PD-1+ CD8+ T cells, which are enriched for tumor-specific clonotypes, were activated at 1 week after SBRT. However, circulating T REG cells were also activated after SBRT; this pattern was also evident among effector Foxp3hiCD45RA− T REG cells. We observed no difference in T-cell responses according to the fraction size and number. Notably, activation of T REG cells was more prominent in patients who experienced greater activation of PD-1+ CD8+ T cells. Plasma level changes in TGF-β1, soluble CTLA-4, and soluble 4-1BB at 1 week after SBRT were associated with PD-1+ CD8+ T-cell responses. Activation of T REG cells at 1 week after SBRT was associated with worse progression-free survival. Clinical factors including molecular subtype were not associated with the T-cell responses. SBRT induced activation of both potentially tumor-specific CD8+ T cells and T REG cells, which were tightly associated with each other. These results may support the use of T REG cell-modulating strategies with SBRT to improve the antitumor immune response. [ABSTRACT FROM AUTHOR]