ERBB2 and breast cancer: Because patients with stage IV HER2+ or triple-negative breast cancer are at particularlyhigh risk of developing brain metastasis and because the prognosis of patients who developbrain metastases is very poor, we investigated whether low dose PCI could reduce theincidence of brain metastasis in a mouse model of metastatic breast cancer, in whichtail-vein injection of HER2+ breast cancer cells led to a high rate of brain metastasis[20].