This Phase III Trial evaluates whether breast conservation surgery and endocrine therapy results in a non-inferior rate of invasive or non-invasive ipsilateral breast tumor recurrence (IBTR) compared to breast conservation with breast radiation and endocrine therapy.

This Phase III trial compares the recurrence-free interval (RFI) among patients with early-stage, low risk HER2+ breast cancer who undergo breast conserving surgery and receive HER2-directed therapy, and are randomized to not receive adjuvant breast radiotherapy versus those who are randomized to receive adjuvant radiotherapy per the standard of care.

This Phase III Trial will determine whether adjuvant chemotherapy (ACT) added to ovarian function suppression (OFS) plus endocrine therapy (ET) is superior to OFS plus ET in improving invasive breast cancer-free survival (IBCFS) among premenopausal, early- stage breast cancer (EBC) patients with estrogen receptor (ER)-positive, HER2-negative tumors and 21-gene recurrence score (RS) between 16-25 (for pN0 patients) and 0-25 (for pN1 patients).