Abstract P1-08-13: Comparison of gene expression profiling results and clinical outcomes among patients with pleomorphic ILC and classic ILC
Authors: Dempster Marcus , Wright Matthew , Cocco Daniela , Elsherif Ayat , Valente Stephanie A , Li Hong , Kruse Megan L 
Published: March 2022

Abstract
Background Invasive lobular carcinoma (ILC) is the second most common histological subtype of breast cancer. Pleomorphic ILC (pILC) is a pathologically defined subtype of ILC that often presents at a higher stage compared to Classical ILC (cILC). pILC is traditionally thought to have poor outcomes compared with cILC. Utility and application of gene expression profiling, such as Oncotype Dx testing, for these patients is unknown. The purpose of this study is to compare Oncotype Dx recurrence scores, treatment patterns, and clinical outcomes for patients with cILC and pILC at our institution. Methods A retrospective analysis of a large institutional cancer database was performed to identify patients with ILC treated between 2004 and 2017. Patient and disease characteristics were collected, including subtype of ILC (classical vs pleomorphic), staging, treatment history, and presence of Oncotype Dx testing. Findings were analyzed for differences in clinical characteristics, Oncotype Dx results, progression free survival (PFS) and overall survival (OS) between patients who received endocrine therapy/chemotherapy (CET) versus endocrine therapy alone (ET) in both cILC and pILC groups. Results: 692 patients with ILC were identified with 100 (14.5%) categorized as pILC. The mean age at presentation was 62 for cILC and 60 for pILC. cILC was more frequently ER positive (98.5% vs. 94.0%, p=0.004). and less likely HER2 positive (7.0% vs 12.2%, p=0.07). pILC more commonly had more advanced tumor stage (Stages II-III 61% vs 41%, p&lt;0.001) but lower nodal stage (N+ 48.5% vs 66%, p&lt;0.001) 25% (25/100) of pILC patients had Oncotype Dx testing performed compared to 33% (198/592) of cILC patients. The majority of patients who had testing had T1cN0 or T2N0 disease (36% and 44% for pILC vs 34%, and 27% for cILC, respectively). A low risk recurrence score (0-10) accounted for 8% (2/25) of pILC cases and 18.2% (36/198) of cILC. Intermediate risk (11-25) accounted for 72% (18/25) of pILC and 73.2% (145/198) of cILC. High risk (26+) accounted for 20% (5/25) of pILC and 8.6% (17/198) of cILC. There was no statistically significant difference in Oncotype Dx recurrence score distribution between pILC and cILC (p=0.117). More patients with pILC received CET compared to cILC (62.0% vs 36.7%, p&lt;0.001). Among pILC cases, 62% received CET and 34% received ET. There was no significant difference in RFS or OS between CET and ET among pILC cases (10-year RFS 74.7% (ET) vs. 76.6% (CET), p=0.52. 10-year OS 92.6% (ET) vs 78.0% (CET), p=0.40). Conclusions At our institution, patients with cILC more often had Oncotype Dx testing compared to those with pILC. Some of the difference may be related to more frequent HER2-positivity in pILC. When testing was done, pILC was twice as likely to have a high risk recurrence score compared to cILC, but this result was not statistically significant. Patients with pILC were more likely to receive CET than patients with cILC. There was no difference in survival between patients with pILC treated with CET versus those treated with ET alone although the sample size is small and this is worthy of further study in a larger population. These results suggest that gene expression testing may be underutilized in pILC.
Citation Format: Marcus Dempster, Matthew Wright, Daniela Cocco, Ayat Elsherif, Stephanie A Valente, Hong Li, Megan L Kruse. Comparison of gene expression profiling results and clinical outcomes among patients with pleomorphic ILC and classic ILC [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-08-13.