Abstract P4-06-04: Clinical outcomes of loco-regional HER2-positive invasive lobular carcinoma of the breast
Authors: Gaire Suman , Budhathoki Pravash , Joshi Utsav , Shah Anish , Choong Grace M. , Yadav Siddhartha 
Published: March 2023

Background: HER2-positive invasive lobular carcinoma (ILC) of the breast is a rare entity. Treatment decisions for HER2-positive lobular breast cancer are often extrapolated from the HER2-positive ductal subtype of breast cancer but the clinical outcomes and prognostic factors are not well defined. Methodology: Women with a diagnosis of stage I to III HER2-positive ILC or invasive ductal carcinoma (IDC) of the breast between 2010 and 2018 were identified from the National Cancer Database. Baseline characteristics were compared between ILC and IDC using Chi-square test for categorical variables and the Mann-Whitney U test for continuous variables. Five-year overall survival (OS) was estimated by the Kaplan Meir method. Multivariate cox proportional hazards regression model including age, race, ethnicity, Charleston Deyo score, grade, TNM stage, and treatment modalities such as type of surgery, chemotherapy, and radiation was used to identify factors associated with OS in HER2-positive ILC. In a subset of patients receiving neoadjuvant chemotherapy, the odds of achieving a complete pathological response were compared between women with ILC and IDC in a multivariate logistic regression model adjusting for age, race, ethnicity, Charleston Deyo score, grade, and clinical stage. Results: A total of 4,197 women with HER2-positive ILC and 116,984 women with HER2-positive IDC were included in the final analysis. The median age at diagnosis was 63 years for women with HER2-positive ILC and 56 years for women with HER2-positive IDC (p&lt; 0.001). The five-year OS among women with HER2-positive ILC was 93.1%, 90.8%,and 78.8% in TNM stages I, II, and III respectively. In multivariate analysis, a significant difference in difference in overall survival was not observed between women with HER2 positive ILC and IDC (Hazard Ratio [HR]: 1.0, 95% Confidence Interval [CI]: 0.9 - 1.1, p=0.55). Among women receiving neoadjuvant chemotherapy, a complete pathological response was observed in 31.7% of women with HER2-positive ILC and 42.7% of HER2-positive IDC. In multivariate analysis, there was no difference in odds of achieving a complete pathological response to neoadjuvant chemotherapy between HER2-positive ILC and IDC (Odds Ratio [OR]: 0.8, 95%CI: 0.7 - 1.0, p=0.12). Higher odds of complete pathological response in HER2-positive ILC were observed for women with estrogen-receptor negative (OR: 2.0, 95% CI: 1.1- 3.8), p= 0.02) and progesterone-receptor negative (OR 2.4, 95%CI: 1.5- 3.7), p&lt; 0.001) tumors. The five-year OS for women with a complete pathological response, partial response, and no response after neoadjuvant chemotherapy in HER2-positive ILC were 89.6%, 84.9%, and 77.3% respectively (p=0.01). Conclusion: This study demonstrates that HER2-positive ILC has comparable clinical outcomes to HER2-positive IDC and response to neoadjuvant chemotherapy correlates with OS in HER2-positive ILC. These findings lend support to the current practice of treating HER2-positive ILC and IDC in a similar manner.

Citation Format: Suman Gaire, Pravash Budhathoki, Utsav Joshi, Anish Shah, Grace M. Choong, Siddhartha Yadav. Clinical outcomes of loco-regional HER2-positive invasive lobular carcinoma of the breast [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-06-04.