The impact of breast radiotherapy on the tumor genome and immune ecosystem

Radiotherapy is a pillar of breast cancer treatment; however, it remains unclear how radiotherapy modulates the tumor microenvironment. We investigated this question in a cohort of 20 patients with estrogen-receptor positive (ER⁺) breast tumors who received neoadjuvant radiotherapy. Tumor biopsies were collected before and 7 days postradiation. Single-cell DNA sequencing (scDNA-seq) and scRNA-seq were conducted on 8 and 11 patients, respectively, at these two time points. The scRNA data showed increased infiltration of naive-like CD4 T cells and an early, activated CD8 T cell population following radiotherapy. Radiotherapy also eliminated existing cytotoxic T cells and resulted in myeloid cell increases. In tumor cells, the scDNA-seq data showed a high genomic selection of subclones in half of the patients with high ER expression, while the remaining number had low genomic selection and an interferon response. Collectively, these data provide insight into the impact of radiotherapy in ER⁺ breast cancer patients.