Metastatic Lobular Breast Cancer: Outcomes of Patients Treated at a Single Tertiary Institution.

[BACKGROUND] Management for lobular breast cancer is generally extrapolated from trials comprised predominantly of ductal breast cancer patients. However, lobular cancers are biologically distinct with associated treatment implications, but despite this, prospective trial data focusing on lobular cancers is lacking.

[METHODS] We conducted a retrospective review of patients diagnosed with metastatic lobular breast cancer at our cancer service. We collected data on clinical characteristics and analysed treatment outcomes following various lines of systemic therapies.

[RESULTS] Between January 2017 and April 2023, 21 patients with metastatic lobular breast cancer were identified. As expected, most (76.2%) patients had a hormone-receptor-positive HER2-negative (HR+HER2-) disease. The mean age at diagnosis of metastatic disease was 60.61 years, and the mean time to progression from early to metastatic disease was 8.45 years. Responses to systemic treatments given for metastatic disease varied significantly. For patients with HR+HER2- disease, the highest clinical benefit rate was observed with capecitabine, followed by a CDK4/6 inhibitor plus a non-steroidal aromatase inhibitor.

[CONCLUSION] This study aligns with existing literature in terms of the clinical characteristics of early lobular breast cancers. For metastatic disease, HR+HER2- lobular breast cancer treated with CDK4/6 inhibitors and non-steroidal aromatase inhibitors had a similar progression-free survival compared to the registration trial data supporting its use for lobular cancer. There were also some responses to chemotherapy, with all patients who received capecitabine obtaining clinical benefit. Patients with HER2+ disease had excellent outcomes, while outcomes were poor for triple-negative lobular breast cancer, though the small number of patients in our study limits interpretation.