Ultrasound Imaging Enables Longitudinal Tracking of Vascular Changes that Correlate with Immune Response After Radiotherapy

Background While immunotherapy shows great promise in patients with triple negative breast cancer, many will not respond to treatment. Radiotherapy has the potential to prime the tumor-immune microenvironment for immunotherapy. However, predicting response is difficult due to tumor heterogeneity across patients, which necessitates personalized medicine strategies that incorporate tumor tracking into the therapeutic approach. Here, we investigated the use of ultrasound (US) imaging of the tumor vasculature to monitor the tumor response to treatment. Methods We utilized ultrafast power doppler US to longitudinally track the vascular response to radiotherapy. We used 4T1 (metastatic) and 67NR (non-metastatic) breast cancer models to determine if US measurements corroborate conventional histological analysis of tumor vessels. To evaluate the effects of radiation, tumor volume and vascular index were calculated using US, and the correlation between vascular changes and immune cell infiltration was determined. Results US tumor measurements and the quantified vascular response to radiation were confirmed with caliper measurements and immunohistochemistry staining, respectively, demonstrating a proof-of-principle method for non-invasive vascular monitoring. Additionally, we found significant infiltration of CD8+ T cells into irradiated tumors 10 days after radiation, which followed a sustained decline in vascular index and an increase in splenic CD8+ T cells that was first observed 1 day post-radiation. Conclusions Our findings reveal that ultrafast power doppler US can evaluate changes in tumor vasculature that are indicative of shifts in the tumor-immune microenvironment. This work may lead to improved patient outcomes through observing and predicting response to therapy. ### Competing Interest Statement The authors have declared no competing interest. * APC : Antigen presenting cell DAPI : 4’,6-diamidino-2-phenylindole dihydrochloride DC : Dendritic cell Gy : Gray IQ : in-phase and quadrature MDSC : myeloid-derived suppresor cell NBF : Neutral buffered formalin RECIST : Response Evaluation Criteria in Solid Tumor RF : Radiofrequency RT : radiotherapy SVD : Single value decomposition TNBC : Triple negative breast cancer US : Ultrasound