Biological Effects of Intraoperative Radiation Therapy (IORT): Histopathological Changes and Immunomodulation in Breast Cancer Patients

Intraoperative radiation therapy (IORT) delivers a single accelerated radiation dose directly to the breast tumor bed. The synergistic effects of simultaneous surgery and radiation are poorly understood. We here explored the cellular and molecular changes induced by IORT in the tumor bed compared to no-IORT exposed breast tissues. Squamous metaplasia (SM) was identified in both cohorts; however, squamous metaplasia with atypia (SMwA) was only seen in IORT specimens (63.6%, p=0.004). SMwA mimicked DCIS on histology, requiring confirmatory stains (i.e., estrogen receptor and p63) for diagnosis. Neither SM nor SMwA were found in the initial BCS specimen. We identified 1,662 differentially expressed genes (875 upregulated and 787 downregulated) between IORT and non-IORT samples. Gene ontology analyses showed that IORT was associated with the enrichment of several immune response pathways, such as inflammatory response, granulocyte activation, and T-cell activation (p<0.001). Notably, when only considering normal tissue from both cohorts, IORT was associated with intrinsic apoptotic signaling (p<0.001). Finally, we inferred a higher abundance of gamma delta T-cells, dendritic cells, and monocytes in the IORT samples compared to non-IORT samples. In conclusion, IORT induces histological changes, including squamous metaplasia with atypia, and elicits molecular alterations associated with intrinsic apoptotic signaling and immune response pathways. The higher abundance of immune-related components in breast tissue exposed to IORT suggests a potential shift towards active immunogenicity in traditionally considered immune-desert tumors such as HR-positive/HER2-negative breast cancer.