Abstract PO3-08-11: Association between Histologic Variants and the Results of Multigene Testing in Patients with Hormone Receptor-positive, HER2-negative Invasive Lobular Cancer

Abstract Invasive lobular carcinoma (ILC), which accounts for approximately 5-15% of all breast cancers, can be divided into classic type and various other variants. Multigene testing (MGT) has prognostic value in hormone receptor (HR)-positive, HER2-negative breast cancer. We aimed to investigate the association between the results of MGT and histological variation of ILC. 1582 HR-positive, HER2-negative breast cancer patients treated between January 2014 and December 2022 were reviewed. Patients were divided into three groups; ILC-favorable group, ILC-unfavorable group and invasive ductal carcinoma (IDC) group. The ILC-favorable group consisted of classic and tubulolobular types, while the ILC-unfavorable group included pleomorphic, solid, signet ring cell, florid, and alveolar types according to the reported prognosis of each ILC variant. Among the various types of MGT, the 21-gene assay (OncotypeDX), the 70-gene assay (Mammaprint), and the 12-gene assay (Endopredict) assay were used in clinical setting. The median age of patients was 50 years (range 20-85 years). 75.5% of patients received the 21-gene assay, 12.8% received the 70-gene assay, and 11.6% received the 12-gene assay. The high-risk results in MGT were showed in 7.7% of 155 patients in ILC-favorable group, 26.1% of 23 patients in ILC-unfavorable group, and 20.9% of 1404 patients in IDC group (p-value < 0.001). In the multivariate logistic regression analysis, ILC-favorable group showed significantly lower odds of high-risk results compared to IDC group (OR=0.284, 95% CI; 0.147-0.548), while ILC-unfavorable group did not show any significant difference compared to IDC group (OR=0.797, 95% CI; 0.263-2.411). In comparison to IDC, the variants of ILC with favorable prognosis show lower rates of high-risk results in MGT, while the variants of ILC with poor prognosis do not differ significantly from IDC. For patients who diagnosed the variants of ILC with poor prognosis, MGT could be useful for prognosis prediction or treatment decision-making. Citation Format: Jee Hyun Ahn, Suk Jun Lee, Seung Hye Yang, Jee Ye Kim, Hyung Seok Park, Seung Il Kim, Byeong Woo Park, Seho Park. Association between Histologic Variants and the Results of Multigene Testing in Patients with Hormone Receptor-positive, HER2-negative Invasive Lobular 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 PO3-08-11.