Abstract P3-08-01: Prognostic performance of EndoPredict in invasive lobular carcinoma

Background: Invasive lobular carcinomas (ILC) account for around 15% of all oestrogen receptor (ER) positive invasive breast cancers. The EndoPredict assay (EPclin) is a multigene classifier to predict the likelihood of distant recurrence in ER-positive, HER2-negative breast cancer patients treated with adjuvant endocrine therapy and has been validated in several retrospective translational studies. However, these validation studies did not include an analysis of the histological subtypes. Here, we investigate the role of EPclin for the prediction of distant recurrence in women with ILC and compare the prognostication to those with invasive ductal carcinoma (IDC) in TransATAC. Methods: Data on EPclin and histological type of tumour were available for 928 postmenopausal women with ER-positive, HER2-negative disease treated with 5 years of tamoxifen or anastrozole. The primary endpoint was distant recurrence and the primary objective was to assess the prognostic value of EPclin for the prediction of distant recurrence in women with invasive lobular carcinoma. Kaplan–Meier and Cox regression analyses were used to determine distant recurrence risk for 0-10 years of follow-up. Likelihood ratio tests were used to assess the prognostic information provided by EPclin. Hazard Ratios (HR) are for a change in 1 Standard Deviation. Results: 141 (15.2%) had ILC, 710 (76.5%) IDC, and the remaining 77 (8.3%) were of different histological type. EPclin provided highly significant prognostic value for distant recurrence in women with ILC (HR=3.33 (2.33-4.77), P Conclusions: This is the first analysis to focus on the role of EPclin for the prognostic assessment of women with ILC. Although numbers of women with ILC in TransATAC were small, EPclin provided highly significant prognostic information and risk stratification for this subgroup of women. 10-year distant recurrence risk in the EPclin low risk groups were similar between ILC and IDC, suggesting that chemotherapy is not indicated, irrespective of tumour type. Our results show that EPclin is informative in women with ILC. Citation Format: Sestak I, Kronenwett R, Denkert C, Cuzick J, Dowsett M. Prognostic performance of EndoPredict in invasive lobular carcinoma [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P3-08-01.