Molecular profiles and treatment recommendations for invasive lobular carcinoma in a real-world prospective breast cancer registry.

e19291 Background: Invasive lobular carcinoma (ILC), the second most common histological breast cancer type, comprises 10-15% of breast tumors. Although ILC is thought to have a low risk of relapse, patient survival data vary. ILC treatment guidelines mirror those for invasive ductal carcinoma (IDC); however, these were extrapolated from trials that included predominantly IDC. The FLEX Registry captures data from real-world breast cancer patients; the current sub-study evaluated molecular profiles and treatment recommendations in ILC. Methods: The FLEX Registry (NCT03053193) includes stage I-III primary invasive breast cancer patients who receive 70-gene signature (70-GS)/80-gene signature (80-GS) testing and consent to full transcriptome and clinical data collection. This sub-analysis includes 335 ILC patients and 2,179 IDC patients enrolled from 2017 to present. 70-GS stratified tumors by risk of distant metastasis [High (HR), Low (LR), Ultralow (UL) Risk], and 80-GS classified tumors by molecular subtype (Luminal, HER2, or Basal type). Genomic and clinical data were compared for ILC and IDC using chi-square or Fisher’s exact test. Results: ILC represented 13% of FLEX cases (n = 335/2752); 81% were lymph node-negative, 99% ER-positive, and 94% HER2-negative. Clinical risk assessment (MINDACT criteria) classified 61% of ILC and 53% of IDC as low risk (p = 0.03). 70-GS risk distribution in ILC (18% UL, 51% LR, 31% HR) differed from IDC (13% UL, 34% LR, 53% HR; p < 0.01). Discordance between 70-GS and clinical risk was greater in ILC (43%) than IDC (30%, p < 0.01). 80-GS results also differed between ILC (98% Luminal, 1.6% HER2, 0.3% Basal) and IDC (85% Luminal, 3% HER2, 11% Basal; p < 0.01). Inclusion of chemotherapy (CT) in treatment plans was associated with 70-GS risk (p < 0.01); treatment plans that disagreed with 70-GS results were associated with discordant clinical risk (p < 0.05). Treatment plans disagreed with 70-GS in 15% of ILC and 12% of IDC total cases (p = 0.08), and more frequently in HR cases (25% in ILC, 14% in IDC; p < 0.01). Conclusions: The FLEX Registry includes patients with ILC consistent with real-world breast cancer frequencies. ILC demonstrated genomic risk and subtype profiles distinct from IDC. Treatment plans largely agreed with 70-GS results; however, discordance was more frequent in HR ILC than IDC. Future studies will evaluate clinical outcomes; however, these results demonstrate the added value of molecular profiling in ILC and the utility of real-world registry data in evaluating uncommon breast tumor types. Clinical trial information: NCT03053193 .