Abstract PO4-15-11: HER2 STATUS AND TUMOR HETEROGENEITY IN INVASIVE BREAST CARCINOMAS: CLINICAL PATHOLOGICAL IMPACT

Abstract Human epidermal growth factor receptor 2 (HER2) is a major player in breast carcinogenesis with a range of 15-20% of cases harboring gene amplification and overexpression turning the tumor eligible to over eight anti-HER2 drugs in the past 15 years. Recently, novel anti-HER2 antibody-drug conjugates (ADCs) treatment was recently approved to metastatic breast cancer bearing some HER2 expression albeit not due to gene amplification. Immunohistochemically classified, by ASCO-CAP Guidelines (2018), as 1+ cases were enrolled in clinical trials and benefited by the addition of ADCs to chemotherapy. Despite being classified as HER2 negative, most tumors exhibit detectable levels of HER2 protein, reinforcing the importance of HER2 expression evaluation. HER-2 intratumoral heterogeneity (ITH) is reported in up to 40% of breast cancers and is defined as the coexistence of subpopulations of tumor cells with different HER2 gene or protein expression. In this scenario, it becomes very important to evaluate the spectrum of HER-2 expression in correlation with pathological factors as ITH is associated with poor prognosis and anti-HER2 resistance. We designed a retrospective linear study to reanalyze all HER2 immunohistochemical core biopsy slides of patients with primary invasive breast carcinomas (IBC) diagnosed and treated at São Paulo Federal University Hospital between 2019 and 2023. Clinical variables (age, laterality, tumor size by ultrasound imaging, BI-RADS) were collected from clinical files. HER2 slides were reviewed by three observers and reported regarding status by ASCO-CAP 2018 guidelines, HER2 heterogeneity and patterns. Statistical analyses were performed with Chi-Square test of independence, Fisher's exact test and Mann–Whitney U test using IBM® SPSS® Statistics (26.0) software. 353 cases were included; the mean age of the patients at the time of diagnosis was 58 years old. 191 cases (54,1%) were left sided and 162 cases (45,9%) were right sided. Regarding BI-RADS report, the major category was 4 (140 cases), followed by 5 (119 cases). Out of the 164 patients who underwent clinical follow-up at our institute, 14 deaths were reported. Concerning pathological features, IBC of no special type was the prevalent histological subtype with 304 cases (86,1%), followed by invasive lobular carcinoma, with 26 cases (7,4%). 296 cases (83,9%) were HER2 negative and 42 (11,9%) were HER2 positive; 15 cases were HER2 2+ (4,2%). Amongst negative cases, 235 were HER2 0+ and 61 were HER2 1+. Considering ITH, 287 cases (81,3%) were HER2 homogenous with 0+ score as the most prevalent, presenting with 232 cases (80%). 66 cases (18,7%) had ITH, with 1+ as the most prevalent primary score, with 34 cases (51,5%), followed by 3+, with 17 cases (25,8%). Not only HER2 positive carcinomas, but also the presence of heterogeneity in HER2 expression was associated with death outcome (p=0,003 and p=0,001, respectively). Also, HER2 positive IBC had larger tumor size compared to HER2 negative carcinomas (mean size of 25,6 mm x 31,3 mm, respectively; p=0,012); HER2 heterogeneity showed no statistical significance when associated with tumor size (p=0,165). We conclude that ITH is prevalent in HER2 expression and should be addressed in pathology reports since it may play an additional role in tumor progression and drug response, especially in the ADCs scenario. Citation Format: Karla Calaça Kabbach Prigenzi, Lisandra Gonzalez Porta Nova, Adilson Monteiro dos Santos Filho, Ana Luiza da Cruz, Lucas de Figueiredo Barbosa, Angela Flavia Waitzberg. HER2 STATUS AND TUMOR HETEROGENEITY IN INVASIVE BREAST CARCINOMAS: CLINICAL PATHOLOGICAL IMPACT 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 PO4-15-11.