Abstract P2-16-26: Mixed invasive ductal and lobular carcinoma (IDC/L) behaves similarly to invasive lobular carcinoma (ILC) with regard to neoadjuvant chemotherapy response and metastatic dissemination
Authors: Nasrazadani Azadeh , Atkinson Jennifer M , Li Yujia , McAuliffe Priscilla F , Jankowitz Rachel C , Emens Leisha A , Tseng George C , Lee Adrian V , Wolmark Norman , Oesterreich Steffi , Lucas Peter C 
Published: March 2020

Abstract
Mixed invasive ductal and lobular carcinoma (Mixed IDC/L) is a rare subtype (3-5%) of invasive breast cancer with elusive pathophysiology. This entity is characterized by a mixed population of both ductal and lobular components within an individual tumor. Few studies have been published to date which compare Mixed IDC/L to invasive ductal carcinoma (IDC) and invasive lobular carcinoma (ILC) from a histopathologic perspective. Available literature on this topic is sparse and has been conflicting with regards to outcomes, and no studies to date describe the response of Mixed IDC/L to neoadjuvant chemotherapy. Patients with ILC have been shown to have lower response rates to neoadjuvant chemotherapy as compared to patients with IDC, which in turn leads to lower rates of successful breast conserving surgery and higher rates of re-excision due to positive margins. We aimed to compare Mixed IDC/L to pure ILC with regards to response to neoadjuvant chemotherapy and the need for repeat surgical intervention.
We identified 26 patients with Mixed IDC/L and 113 patients with ILC who received neoadjuvant chemotherapy at our institution between 1990 – 2017. At baseline, the groups had a similar median age, as well as ER H-score, PR H-score, and Ki-67 index. There was no statistical difference in rates of pathologic complete response (pCR) or percent tumor volume reduction post therapy between the two groups. Similarly, the percent of patients that required re-excision was not statistically different. Interestingly, the metastatic pattern was similar between the groups and included sites of dissemination such as the peritoneal cavity and omentum, which are not common sites of metastasis for IDC.
These findings suggest that Mixed IDC/L tumors behave similarly to ILC with regard to their response to neoadjuvant chemotherapy and patterns of metastatic spread. These findings support a prominent role for the lobular component in this mixed subtype in driving biology, including response to neoadjuvant therapy and metastatic dissemination. Ongoing efforts are directed towards incorporating data from the IDC cohort, as well as evaluation of changes in histology, ER/PR H-scores, and Ki-67 levels as a result of therapy. These data may imply that Mixed IDC/L tumors may behave clinically more like ILC than IDC, but larger studies are needed to study this rare breast cancer subtype.
Citation Format: Azadeh Nasrazadani, Jennifer M Atkinson, Yujia Li, Priscilla F McAuliffe, Rachel C Jankowitz, Leisha A Emens, George C Tseng, Adrian V Lee, Norman Wolmark, Steffi Oesterreich, Peter C Lucas. Mixed invasive ductal and lobular carcinoma (IDC/L) behaves similarly to invasive lobular carcinoma (ILC) with regard to neoadjuvant chemotherapy response and metastatic dissemination [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P2-16-26.