52P Clinical characterization of HER2 low in patients with lobular breast cancer (ILC)

The antibody-drug conjugate (ADC) trastuzumab deruxtecan is not only efficient in HER2 amplified breast cancers (BC), but also BC with lower protein expression levels of HER2. These low tumors are well described for non-special type BC, data on ILC lacking. Here, we aim at reporting the prevalence and analyzing its association clinicopathological features survival patients early pure estrogen receptor-positive, HER2-negative (ER+/HER2-) ILC. A multicentric retrospective study was performed including diagnosed stage I-III ER+/HER2- between 01/01/2000 12/31/2020. HER2- disease categorized by immunohistochemistry (IHC) score into 0, 1+ 2+ (without amplification) following local guidelines prior to 2007 ASCO/CAP from onwards. variables assessed using multinomial logistic regression. Univariable multivariable analyses were evaluate disease-free (DFS), distant recurrence-free (DRFS) overall (OS). Data 2098 collected which 1103 (53%) had a tumor. Of these, 716 (34%) an IHC 387 (18%) 2+. histological grade 3 (odds ratio (OR) 1.95, p-value 0.04) tumor size ≥2cm (OR 1.85, <0.01) associated vs. 0. This seen In analyses, worse DFS (hazard (HR) 1.29, 0.03) OS (HR 1.41, 0.01) as compared 2 significantly 1.45, 0.04), while trend DFS. No significant results when comparing categories DRFS. our cohort, present more than half ER+HER2- ILCs. Higher scores unfavorable prognostic (higher size) outcomes (DFS OS). Studies needed whether ER+ would benefit anti-HER2 ADCs.