Within a cohort of HER2-positive breast cancer patients treated with concurrent trastuzumab and Hypo-RT or Conv-RT, we note three main findings: (1) No symptomatic cardiac toxicity occurred; (2) Rates of significant asymptomatic LVEF decline in both groups were similarly low and within the range reported by randomized trials with trastuzumab; and (3) Development of asymptomatic LVEF decline was independent of multiple dosimetric cardiac parameters including mean heart dose. This evidence concerns the gene ERBB2 and breast cancer.