Cardiac safety analysis of the first stage of NSABP B-31, a randomized trial comparing the safety and efficacy of doxorubicin and cyclophosphamide (AC) followed by paclitaxel (T) to that of AC followed by paclitaxel plus trastuzumab (TH) in patients (pts) with operable, node-positive (N+), HER-2 overexpressing breast cancer (HER2+BC)