A Study of Cyclophosphamide, Methotrexate, Fluorouracil (CMF) for People With Breast Cancer

Inclusion Criteria: * Patients must have histologically confirmed adenocarcinoma of the breast confirmed at MSK. The patient must have had diagnostic biopsy or completion of primary surgical management, within 3 months of enrollment. Results of HER-2/neu, estrogen receptor, and progesterone receptor are required for study entry. * Patients must be ≥ 70 years of age. * Patients must have ECOG performance status \>2 (Karnofsky score of ≥ 60%). * Patients may have received hormonal therapy for the purpose of chemoprevention. * Adjuvant/neoadjuvant chemotherapy decision to treat based on the discretion of investigator * Cohort 1 only: Patients have any histology except Her 2 Neu positive in the adjuvant or neoadjuvant setting * Cohort 2 only: Patients have triple negative (ER- (10%) PR-Her 2 Neu -) in the neoadjuvant setting * Cohort 2 only: Patients have received at least 1 dose of carboplatin/paclitaxel +/- pembrolizumab in the neoadjuvant setting Required Organ Function Adequate hematologic function defined as follows: * Absolute neutrophil count (ANC) ≥ 1,500 cells/mm3 * Platelets ≥ 100,000 cells/mm3 Adequate renal function defined as follows: ° Creatinine clearance (CrCL) of \>30 mL/min by the Cockcroft-Gault formula: CrCl (mL/min) = \[(\[140 - age(years)\] x weight (kg))/72 x creatinine (mg/dL)\] (x 0.85 for female patients) Adequate hepatic function defined as follows: * Total bilirubin ≤ 1.5 x institutional upper limit of normal (ULN) (patients with known Gilbert's disease who have bilirubin level ≤ 3 x institutional ULN may be enrolled) * AST and ALT ≤3 x institutional ULN * Patients or their Legally Authorized Representative must give written, informed consent indicating their understanding and willingness to participate in the study. Exclusion Criteria: * Stage IV breast cancer * CARG score- high Frail 4 or more as assessed by CAIT * Her 2 Neu positive 3+/amplified breast cancer * No prior chemotherapy within 1 year of study enrollment * Cohort 2 only: patients may not receive pembrolizumab during dd CMF treatment