 Some breast cancer cells make a protein called Human Epidermal Growth factor receptor 2 HER2. Patients with HER2 positive HER2 breast cancer receive medicine that attacks HER2, which helps these patients live longer. Some HER2 negative HER2- breast cancer patients also benefit from medicines that attack HER2, but we do not know why or which patients will benefit. This study uses a new imaging method, HER2-targeted PETCT, to identify patients that may benefit from medicines that attack HER2. This is experimental. 