Abstract Background: Little is known regarding mixed ductal/lobular histology subtypes of breast cancer in terms of how they influence the likelihood of pathologic complete response (pCR) following neoadjuvant chemotherapy (NAC) 0.001). On adjusted analysis, the difference in pCR between patients with mixed 0.001), use of immunotherapy (p0.001), and absence of LVI (p0.001). For OS, larger tumor size (p0.001), positive nodal status (p0.001), 0.001) were associated with worsened OS whereas the receipt of endocrine therapy (p=0.003) significantly improved OS. Histology subtype was not significantly associated with differences in OS (p=0.238). Conclusion: Tumors of mixed ductal/lobular histology behave more similarly to ductal disease in terms of NACI response, including ER-low subgroups. Additional factors, such as tumor size, LVI, and receipt of immunotherapy play a stronger role in predicting pCR, but the histology subtype remains an important variable to consider when deciding how best to sequence treatment for those with HER2-negative ER-expressing disease. Citation Format: Kai C. Johnson, Brandon Slover, Yengeniya Gokun, Dionisia M. Quiroga, Sagar Sardesai, Sachin Jhawar, Nerea Lopetegui-Lia, Arya M. Roy, Gilbert Bader, Ashley P. Davenport, Nicole O. Williams, Robert Wesolowski, Margaret E. Gatti-Mays, Daniel G. Stover, . Impact of histology subtypes (ductal, lobular, and mixed ductal/lobular) on pathological complete response (pCR) following neoadjuvant chemotherapy (NAC) and chemoimmunotherapy (NACI) for estrogen receptor low (ER-low) HER2-negative breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6646.
Johnson et al. (Fri,) studied this question.