HER2-Positive Lobular Versus Ductal Carcinoma of the Breast: Pattern of First Recurrence and Molecular Insights.

Background: Infiltrating lobular carcinoma (ILC) represents about 10% of breast cancer and rarely shows overexpression of human epidermal growth factor receptor 2 (HER2). We compared biological and clinical characteristics of HER2-positive ILC versus HER2-positive infiltrating ductal carcinoma (IDC).

Patients And Methods: We retrospectively analyzed the data of 328 patients with HER2-positive pure ductal or lobular breast carcinoma, comparing clinical and biological data at diagnosis as well as outcome between the 2 histologies. A gene-mutation analysis was performed in a subset of patients.

Results: Two hundred ninety-one patients (88.7%) had IDC and 37 patients (11.3%) ILC. ILC resulted more frequently in multicenter (24.3% vs. 6.5%, P < .0001) and node-positive (54.1% vs. 45%, P = .013) disease of lower proliferative activity (Mib1 < 20%: 51.4% vs. 22.3%, P < .0001) and lower histologic grade (grade 3: 32.4% vs. 57.4%, P = .038). Disease recurred in 57 patients (17.4%) and involved the bone in 40% of ILC patients (vs. 17% of IDC patients) and the viscera in 30% of ILC patients (vs. 59.6% of IDC patients). No difference in the recurrence rate between the 2 histologies was observed in patients treated with adjuvant trastuzumab (12.5% of ILC patients and 8.3% of IDC patients). Exploratory molecular analysis revealed a higher frequency of mutations in ILC, with more cases of multiple mutations.

Conclusion: HER2-positive ILC shows different biological behavior than IDC, with a possible higher mutation load. Despite lower proliferation activity and estrogen receptor expression in ILC breast cancer, trastuzumab is clearly an effective therapy for this histologic subtype.