This prospective study recruits patients with ER+/HER2-, non-metastatic breast cancer who omit upfront surgery in favor of primary endocrine therapy for sample collection and prospective circulating tumor DNA (ctDNA) measurement to guide disease surveillance.

Age is one of the most important risk factors for the development of breast cancer. Nearly a third of all breast cancer cases occur in older women (aged ≥ 70 years), with most cases being estrogen receptor-positive (ER+). Such tumours are unlikely to be the ultimate cause of death for older women, particularly when considering other comorbidities. Our group has recently shown that risk of breast cancer-related mortality is only around 3% for older women who forego upfront surgery and instead treated with primary endocrine monotherapy. However, with omission of surgery, surveillance for tumor growth or treatment failure is important but requires patients (and their caregivers) to have regular imaging and to see their oncology team every 3 months, which can be quite burdensome. Thus, this study employs a ctDNA surveillance scheme to monitor for potential tumor growth. Partnering with Natera, this study uses a bespoke, tumor-specific ctDNA assay (Signatera) to monitor for changes in ctDNA dynamics while the patient is on endocrine therapy.

This study is for patients who choose not to undergo primary surgical intervention and are managed medically on primary endocrine therapy (any endocrine therapy permitted). Patients will undergo Signatera testing at the time of regularly scheduled clinic follow-up visits and/or at the time of surveillance imaging, which will occur every 3-6 months. Patients will also be asked to complete regular surveys while participating in the study.

Inclusion Criteria:

• early-stage (stages I-III) breast cancer
• estrogen receptor positive (ER+) and/or progesterone receptor positive (PR+) and HER2 receptor negative
• able to provide blood samples
• decision made by treating physician and patient to forego upfront surgery in favor of medical management with endocrine therapy
• non-metastatic disease at diagnosis (patients need not have staging scans; patients at higher risk for de novo stage IV disease should not have any symptoms concerning for metastatic disease)
• patients may be taking any endocrine therapy
• patients may be taking any CDK4/6 inhibitor as deemed necessary by their treating physician
• any ECOG Performance Status allowed
• if patients have synchronous bilateral ER+ breast cancer, tissue from both sites should be submitted to Natera to perform ctDNA testing
• patients with multifocal/multicentric tumors are eligible and the largest focus of cancer should be submitted for testing

Exclusion Criteria:

• under 70 years of age
• breast cancer that is not estrogen receptor positive (ER+) or progesterone receptor (PR+)
• unable to provide blood samples or insufficient tumor tissue for Signatera testing

Women only with ER+ breast cancer

Women 70 years of age or older with ER+/HER2-, non-metastatic breast cancer who forego upfront surgery in favor of primary endocrine therapy.