Longitudinal Tumor Burden Quantification Using Circulating Tumor DNA in Metastatic Lobular Breast Cancer

Inclusion Criteria: 1. Signed informed consent 2. Patients must have histologically or cytologically confirmed invasive lobular breast cancer that is ER+ (\> 1% staining) and HER2-negative as per ASCO/CAP guidelines with radiographical or clinical evidence of metastatic disease 1. Lobular histology as assessed on either tissue collected from a metastatic lesion or from the patient's primary breast tumor (in case of recurrent metastatic disease) 2. Patients with mixed ductal/lobular (NST/ILC) tumors are eligible to participate (with the ultimate goal to evaluate 20 patients with pure ILC) 3. Patients must have tumor tissue available for whole exome sequencing for Signatera assay design 3. Prior therapies: 1. Patients must not have received any therapy in the metastatic setting 2. Patients could have received adjuvant therapy as indicated for their primary breast cancer 4. Age ≥ 18 years 5. Patients may be pre- or post-menopausal. Exclusion Criteria: 1. Stage I-III breast cancer 2. Lack of lobular histology on tumor tissue biopsy 3. Other active cancer (previously treated cancer with no current evidence of disease is allowed) 4. ctDNA assay development is unattainable due to insufficient tumor tissue or sequencing failure