This is a prospective observational registry for patients with invasive lobular carcinoma (ILC) of the breast. After the first 1cohort is enrolled, the collected data will be utilized to develop an ILC specific response assessment tool.

Primary Aims:

1. To evaluate the feasibility of developing a histologic based registry for participants living with metastatic lobular breast cancer.
2. To understand the natural history, treatment patterns, and overall survival in patients with metastatic ILC using modern, real-world data.

Secondary Aims:

1. To evaluate the correlation between imaging findings and disease progression.
2. To evaluate the correlation between ctDNA and disease progression.
3. To develop an ongoing platform for evaluating new imaging tools, tumor markers, and participant recruitment for clinical trials.

Outline:

Participants will be recruited to participate in the registry during a regularly scheduled clinic visit with their treating oncologist, and consented to participate at this time. At each evaluation time point, participants will have a blood draw performed. Participants will be followed until loss to follow up, death, or withdrawal from the registry.

Inclusion Criteria:

1. Histologically confirmed invasive lobular carcinoma. Mixed lobular/ductal cancer is allowed.
2. Age >=18 years
3. Any receptor subtype.
4. Ability to understand a written informed consent document, and the willingness to sign it.

Exclusion Criteria:

1. Stage I-III breast cancer.
2. Lack of lobular histology on tumor biopsy.
3. Other active cancer (prior treated cancer with no current evidence of disease is allowed).

Adult participants with histologically proven stage IV invasive lobular carcinoma