• • The patient or a legally authorized representative must provide study-specific informed consent prior to pre-entry/Step 1 and, for patients treated in the U.S., authorization permitting release of personal health information.

‣ The patient must have an ECOG performance status of 0 or 1.

⁃ The patient must have undergone a lumpectomy and the margins of the resected specimen or re-excision must be histologically free of invasive tumor and DCIS with no ink on tumor as determined by the local pathologist. If pathologic examination demonstrates tumor at the line of resection, additional excisions may be performed to obtain clear margins. (Patients with margins positive for LCIS are eligible without additional resection.)

⁃ The tumor must be unilateral invasive adenocarcinoma of the breast on histologic examination.

⁃ Patient must have undergone axillary staging (sentinel node biopsy and/or axillary node dissection).

⁃ The following staging criteria must be met postoperatively according to AJCC 8th edition criteria:

⁃ By pathologic evaluation, primary tumor must be pT1 (less than or equal to 2 cm).

⁃ By pathologic evaluation, ipsilateral nodes must be pN0. (Patients with pathologic staging of pN0(i+) or pN0(mol+) are NOT eligible.)

⁃ Oncotype DX Recurrence Score of less than or equal to 18 on diagnostic core biopsy or resected specimen.

‣ \*\* For patients with a T1a tumor (less than or equal to 0.5 cm in size) or patients at Canadian provinces or approved international sites where Oncotype DX Recurrence Score testing would not be covered, who do not already have an Oncotype DX Recurrence Score at pre-entry/Step 1, a specimen (unstained blocks or slides) must be sent to the Genomic Health centralized laboratory. Tumor size sample must be greater than or equal to 0.2 cm for analysis.

‣ \*\*\* The Oncotype RS can be run on the biopsy core or surgical specimen. The patient cannot have initiated endocrine therapy prior to tissue collection.

⁃ An Oncotype RS is required for eligibility, however, for a patient whose tumor has already had a MammaPrint test completed as part of usual care when being considered for enrollment and is in the binary Low category will meet this eligibility criteria and an Oncotype RS does not need to be performed.

⁃ The tumor must have been determined to be ER and/or PgR positive assessed by current ASCO/CAP Guideline Recommendations for hormone receptor testing. Patients with greater than or equal to 1% ER or PgR staining by IHC are considered positive.

⁃ The tumor must have been determined to be HER2-negative by current ASCO/CAP guidelines.

⁃ Patients may be premenopausal or postmenopausal at the time of pre-entry/Step 1. For study purposes, postmenopausal is defined as:

⁃ Age 56 or older with no spontaneous menses for at least 12 months prior to pre-entry/Step 1; or a documented hysterectomy; or

⁃ Age 55 or younger with no spontaneous menses for at least 12 months prior to pre-entry/Step 1 (e.g., spontaneous or secondary to hysterectomy) and with a documented estradiol level in the postmenopausal range according to local institutional/laboratory standard; or Documented bilateral oophorectomy.

⁃ The interval between the last surgery for breast cancer (including re-excision of margins) and pre-entry/Step 1 must be no more than 70 days.

⁃ The patient must have recovered from surgery with the incision completely healed and no signs of infection.

⁃ Bilateral mammogram or MRI within 6 months prior to pre-entry/Step 1. HIV-infected patients on effective anti-retroviral therapy with undetectable viral load within 6 months are eligible for this trial. Patients must be intending to take endocrine therapy for a minimum 5 years duration (tamoxifen or aromatase inhibitor). The specific regimen of endocrine therapy is at the treating physician's discretion.