Testing Low Dose Tamoxifen for Invasive Breast Cancer, LoTam Trial

Inclusion Criteria: * Unilateral invasive adenocarcinoma of the breast that is histologically confirmed * Invasive breast cancer is estrogen receptor positive in ≥ 10% of cells * HER2 negative by current American Society of Clinical Oncology (ASCO)/College of American Pathologists (CAP) guidelines * The patient must have a multigene assay with a low-risk score, including any of the following (if more than one genomic assay was obtained, both are required to be low-risk): * Oncotype DX recurrence score ≤ 25 * Mamma Print low risk * Prosigna risk of recurrence ≤ 40 Patients with pathologic tumor size ≤ 0.5cm who meet all other eligibility criteria are eligible for enrollment without multigene assay testing. The total number of enrolled patients with tumor size ≤ 0.5cm without genomic testing will be limited * Tumor size must be ≤ 3 cm by pathologic evaluation. Multifocal tumors are permitted if the largest focus of disease is ≤ 3 cm * Adequate surgical removal of all clinically evident disease in the breast with either breast conserving surgery or mastectomy. Negative margins on final pathology are required. Additional excisions may be performed to obtain clear margins before registration * No clinical (cN1, cN2, cN3) or pathologic (pN1mi, pN1, pN2, or pN3) evidence of lymph node involvement on either needle biopsy or surgical lymph node assessment. Patients with pN0(i+) or pN0 (mol+) are eligible * Surgical axillary staging (sentinel lymph node biopsy ± axillar…