Abstract PO5-01-01: The comparison of HER2 low and HER2 0 in ductal carcinoma in situ (DCIS) for breast cancer

Abstract Background: Trastuzumab deruxtecan is a standard of care after prior chemotherapy for metastatic breast cancer with HER2 low. HER2 low recognized as a new category of breast cancer. The definition of HER2 low is IHC 1+ or IHC 2+/ISH- in invasive ductal components. We have already reported the frequency and prognosis of HER2 positive and low in invasive cancer (Breast Cancer. 2022;29:234-241), but the distribution and characteristics of HER2 low in ductal carcinoma in situ (DCIS) is unknown. It is also well known that HER2 expression is higher in DCIS than in invasive cancer. We focused on the frequency of HER2 low in DCIS, performed an analysis of its frequency and characteristics, and compared to DCIS with HER2 0. Methods: We retrospectively examined breast cancer patients with DCIS and invasive ductal carcinoma (IDC) who underwent surgery at our institution between 2005 and 2015. Regarding the expression status of HER2, we classified cases into three groups: HER2 0, HER2-low (defined as IHC 1+ or IHC 2+/ISH-), and HER2-positive (IHC 2+/ISH+ or IHC 3+), and investigated their frequency. We analyzed the relationship between HER2 expression and Van Nuys classification, as well as the relationship between HER2 expression and ER expression, using the chi-square test. Results: A total of 726 DCIS patients were included in this analysis. The patient demographics showed a median age of 52 years (range: 20-84), with 413 patients (56.9%) undergoing total mastectomy and 311 patients (42.8%) undergoing breast-conserving surgery (two cases had unknown surgical procedures). The expression of HER2 in DCIS patients was observed as HER2-0, low and positive in 100 (13.8%), 474 (65.3%) and 152 patients (20.9%), respectively. The expression of HER2 in IDC was observed as HER2 0, low and positive in 838 (17.1%), 3169 patients (64.4%) and 911 patients (18.5%) (Table 1). DCIS tended to have higher HER2 expression(P=0.048). Then we compared the characteristics of HER2 0 and HER2 low in DCIS. Table 2 shows the relation between Van Nuys classification and HER2 expression. Van Nuys classification was evaluated in 346 cases. Van Nuy grade 3 accounted for 4.8% of the HER2 0 cases and 15.8% of the HER2 low cases. HER2 low cases tend to have higher grades(P 0.01). Table 3 shows the relation between ER expression and HER2 expression. ER negative accounted for 5% of HER2 0 cases and 8.4% of HER2 low cases. ER negatives tended to be more common in Her2 low cases(P 0.01). Conclusion: HER2 low is not considered a molecular biological classification, but rather a classification of the target of treatment for Trastuzumab deruxtecan. In our study, comparing HER2 0 and HER2 low, HER2 low tended to have higher grade and lower ER expression. Further investigation including basic research is needed to clarify the biological meaning of her2 low. Table1: The frequency of HER2 expression in DCIS and IDC Table2: The relationship between Van Nuys classification and HER2 expression Table3: The relationship between ER status and HER2 expression Citation Format: Nari Kureyama, Maho Kusudo, Akira Nakakami, Rie Komaki, Yuka Endo, Kazuki Nozawa, Ayumi Kataoka, Haruru Kotani, Akiyo Yoshimura, Masaya Hattori, Masataka Sawaki, Hiroji Iwata. The comparison of HER2 low and HER2 0 in ductal carcinoma in situ (DCIS) for breast cancer abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-01-01.