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

This study is tracking circulating tumor DNA (ctDNA) — fragments of DNA shed by cancer cells into the bloodstream — in people with metastatic lobular breast cancer (a specific type of ER-positive, HER2-negative breast cancer known for being harder to detect on scans) to see if blood tests can monitor how well treatment is working over time. **You may be eligible if...** - You are 18 or older - You have been diagnosed with invasive lobular breast cancer (or mixed ductal/lobular) that is ER-positive and HER2-negative - Your cancer has spread to other parts of the body (metastatic) but you have NOT yet started any treatment for the metastatic stage - Tumor tissue from your breast cancer is available for genetic analysis **You may NOT be eligible if...** - Your breast cancer is still in an early, non-metastatic stage (Stage I-III) - Your tumor does not have lobular features on biopsy - You have another active cancer at the same time - Sufficient tumor tissue is not available for genetic testing Talk to your doctor to see if this trial is right for you.