ERBB2 and cancer: In this work, we used a unique experimental system – a robust mouse model of Her2-positivebreast cancer brain metastasis and a dedicated small-animal irradiator –to address thepotential efficacy of PCI, which may be clinically relevant for patients with breast cancer.The median survival time for patients with breast cancer and brain metastases is well under1 year, and patients with stage IV HER2+ or triple-negative breast cancer are atparticularly high risk of developing brain metastases.