HER2-positive invasive lobular carcinoma: a rare breast cancer which may not necessarily require anti-HER2 therapy. A population-based study.

BACKGROUND: HER2-positive (HER2 +) invasive lobular breast cancer (ILC) is rare and poorly characterised. In particular, patient outcomes compared to those associated with HER2 + invasive ductal cancer (IDC) and HER2-negative (HER2 -) ILC, as well as the benefits of anti-HER2 therapy, are not well established. METHODS: We analysed the data from the Côte d'Or Registry of Breast and Gynaecological Cancers (France) for all patients diagnosed with early-stage HER2 + ILC (62 cases), HER2 + IDC (833 cases) and HER2 - ILC (685 cases) between 1998 and 2015 to compare overall and disease-free survival (OS and DFS) between these groups in correlation with anti-HER2 therapy. RESULTS: ILCs were associated with older age, larger tumours, lower histological grades, higher hormonal receptor positivity rates and multifocality, and more common endocrine therapy. OS and DFS between the three groups did not differ. We found that anti-HER2 therapy was associated with a survival benefit in patients with HER2 + IDC. In contrast, the survival of HER2 + ILC patients was not improved by anti-HER2 treatment, remaining close to that of HER2 - ILC patients. CONCLUSION: HER2 + ILC seems not to be associated with better outcomes than HER2 + IDC but may not differ from HER2 - ILC in terms of survival.